Version: 0.5.9
No additional mods have been added and no config files have been edited. The hardware it is running on is my personal desktop, running Windows 10 and Java 8.

At seemingly random intervals, ranging from once every several minutes to several times a second, every player currently logged into the server immediately has their coordinates rounded down to the nearest whole number, resulting in their teleportation to the Northwestern corner of whatever block they are standing on.

It's like the player positions are being cast as integers from floats and it's causing a loss of precision. It is wildly inconsistent as to when it happens, though it is frequent enough to be extremely irritating, but it is completely consistent in the method that the teleportation occurs.

The issue does not appear in either the normal log or the debug log, other than the occasional message that a player "moved wrongly". This "moved wrongly" message only rarely appears and may or may not be directly linked to the bizarre teleportation issue.

For my server, the issue is frequent enough that I cannot imagine anyone else could be seeing it without it being an obvious issue, so I'm more than happy to provide any additional information about my specific setup if needed.

Im getting the same issue too, so its not just your setup!

i haven't had it happen to players on my last few playthrougs, but i did notice it happen to mobs released from imprisonment tools or using them in the essence spawner (them appearing in corners before being shoved to the center), not sure if its related to this issue but it rang a bell 🤔

Thank you for the report, I will investigate the issue.

i'd like to comment that it did seem to happen to me today, i was playing alone on the server & got teleported to a corner as soon as another player seemed to join 🚪

After a bit more testing, I have also seen it happen to non-player entities. The cat by my waystone visibly teleported to the northwest corner of the block it was in when I arrived at the stone.

teleport events seem to coincide with this red message in the server console: (still not sure what triggers it though)

[Netty Epoll Server IO #9/ERROR] [minecraft/ArgumentTypes]: Could not serialize dev.maxneedssnacks.interactio.command.RegistryArgument@3d4588fa (class dev.maxneedssnacks.interactio.command.RegistryArgument) - will not be sent to client!

I'm not 100% sure, but I do not believe that it is related to that message. At the very least, it is not a 1:1 correlation as I can confirm it definitely does not show up in the console every time the teleportation occurs. I believe it just might look that way because that console error happens frequently enough that it just happens to be in the log when you are going to look for it.

I have definitely noticed a few times that the coordinate rounding can happen when someone uses a waystone, but since it also definitely happens without the waystone activating I suspect that it's something happening at a more fundamental level. The only mod that I see that obviously seemed to interact at a basic level with player movement is randompatches, since it tweaks the player movement caps. I tried removing it from the pack but can confirm that it isn't related to the issue as the rounding still happened without it installed.

Also, just to be safe I tried running the server both with and without allow-flight enabled in server.properties and it had no impact. I'm going to go and see if the issue shows up in any other modpacks, since I highly doubt that this would be an issue completely specific to just this particular combination of mods and I don't think the Enigmatica core mod would be causing the issue. If I can find it happening somewhere else, I hopefully can get a list of what mods in that pack overlap with this one and narrow down the search a bit.

Also, I haven't done any serious modding since Forge was just replacing Risugami's Modloader so I'm not up-to-date on the intricacies of the current build process, is there a good way to get other mods working with a Forge development setup without running into the issue of the obfuscation of the added mods breaking it? This would likely be easier to debug if I could get in there and set some breakpoints, but I don't know that it's possible currently and haven't had the time to experiment too much.

teleport events seem to coincide with this red message in the server console: (still not sure what triggers it though)

[Netty Epoll Server IO #9/ERROR] [minecraft/ArgumentTypes]: Could not serialize dev.maxneedssnacks.interactio.command.RegistryArgument@3d4588fa (class dev.maxneedssnacks.interactio.command.RegistryArgument) - will not be sent to client!

That message happens when a player joins the server.

the Enigmatica core mod

The what now? :P

Ack, sorry, I meant the Companion Mod.

Core, Companion, while they may be opposite words from one another they are obviously the same thing because if you squint they look kind of similar. :P

I found a way to replicate the issue. This requires two players
Player A has to stand somewhere waiting for the bug while Player B needs to be outside of the view distance of Player A and then reenter the view distance (teleporting into it is the easiest method) and the bug happens.

Would you mind posting your debug.log @DraGonFiirE ? :)

DraGonFiirE, that is exactly what I had thought was happening but wasn't able to get my spastic friends to test it properly. I'll try again and see if I can confirm it.

This is happening to myself and a couple of friends, we were able to replicate it reliably using @DraGonFiirE's method. I've uploaded my debug.log here: https://www.dropbox.com/s/rfq4wjzkucrbfdt/debug.log?dl=0

Thanks! There's nothing helpful in there unfortunately.
But fyi the message [com.robotgryphon.compactcrafting.CompactCrafting/]: Found 0 nearby projectors. is spammed 100.000+ times

That looks unrelated, and it seems the devs over at compact crafting are aware of it CompactMods/CompactCrafting#32

An even easier way to reproduce it is to just place an armor stand and it spawns at the northwest corner and then snaps to the center of the block

An even easier way to reproduce it is to just place an armor stand and it spawns at the northwest corner and then snaps to the center of the block

as do mobs released from imprisoning tools or spawned (by a mob duplicator)

An even easier way to reproduce it is to just place an armor stand and it spawns at the northwest corner and then snaps to the center of the block

as do mobs released from imprisoning tools or spawned (by a mob duplicator)

I can confirm that this issue persists into version 0.5.11. As we are playing on a server and other players are exploring and entering/leaving the Nether, this happens quite often. I am quite certain that this does not happen when mobs spawn naturally or when moving Entities (projectiles like Arrows) are created.

Could this also be a client-sided issue? For as far as I know, non-player entities only get snapped to the grid visually for a moment and then get "corrected", right?
And the movement of the players is controlled from the client. The server only corrects it if it is moving too fast in a certain direction. I think the server allows uncontrolled teleportation within a block of distance every so often.

I've also noticed that this rounding happens if you try to shrink yourself with a personal shrinking device that has no energy. I was on a server and I started spamming it, and it kept rounding my position to the northwest, so if I faced that way and moved forward slightly, it would cause me to move super fast in that direction. (This was also 0.5.11 btw)

Player A has to stand somewhere waiting for the bug while Player B needs to be outside of the view distance of Player A and then reenter the view distance (teleporting into it is the easiest method) and the bug happens.

Can confirm that it doesn't have to be teleporting at all. Another player in a boat moving towards me did the trick quite convincingly. And this further supports this being a client-sided issue. If it were a server-sided issue, the rounded teleportation would probably happen as soon as the area's of chunks loaded by the player entities touch or start to overlap. Instead it happened when they got within render distance.

looks like the shrink mod actually was the culprit all along:

gigabit101/Shrink#52
FTBTeam/FTB-Modpack-Issues#72

i have tested a fix suggested by someone from the above links & it seems to work fine in enigmatica 6, now we just have to wait for gigabit101/Shrink#53 to be merged, but the mod author doesn't seem too active or concerned with it, and i doubt a custom version can be used in the pack nor could the mod be removed while it causes teleportation motion sickness? 🤔

We can't use a custom build of it, no :/