Single player.

No crash, just can't load my home's chunks.

Any time I go to another dimension like the Nether or End, I notice that my Ender Bag won't clear according to machines in my supposedly chunk loaded space back in the Overworld. If I'm exploring somewhere in the Overworld, however, the Ender Bag clears with few issues, provided I am close enough. I use Pipez to clear the associated Ender Chest into a buffer for processing ore and such. But while using a Quarry to mine the ATM mining dimension for ore, that particular Ender Chest still fills with ore yet to be processed no matter where I am, indicating that this other chunk loading works fine. I have tried several times to unclaim the chunks, claim them again, and set them to force load to no avail. I'm considering looking for a persistent, block-based chunk loader but don't want to overload my game client. If it helps, I settled within one of those random, rogue-like dungeons in the Overworld. Maybe the spawners or other architecture are preventing the FTB mod from loading my chunks? I have no idea.

Can confirm same issue occuring, set the area surrounding my ME and power to remain chunk loaded, head to nether/end/moon etc. It remains accessible from a wireless crafting terminal and then after about a minute I loose connection to my ME until I return back home. Same issue with ender bags etc, so seems that the chunk loader is ignoring requested chunks to remain loaded.

I'm having the same issues on my server on version 2.44. I know ATM 6 had this issue briefly with the Performant mod and "fastChunkLoading" nullifying the forceloaded chunks from working. I thought I was going crazy because I setup something in the compact machines dimension and just gave up and set it up in the overworld close to my base. I just set up a quarry in the mining dimension and the loaded chunks stay loaded for about 30 seconds and then everything stops working. I'm about to just set up a bigger reactor in the mining dimension and just idle there. Super lame. Also I'm pretty sure compact machines are supposed to be force loaded by default so that isnt working either.

Thinking about it more but no matter where you are in the dim the loaded chunks work fine for that dimension only.

I'd like to add that I was using the Tesseract for the tesseract mod and I switched to using the Entangled block and bounding an Integrated Dynamics Battery in the overworld to the entangled block in the mining dim and everything started working. Maybe the entangled block does its own chunk loading? That setup works for some reason.

Issue also happening here.
I can confirm that as long as 1 person is in the dimension that you are chunk loading in - it functions. I can have a friend far away from the main base where RS system and ender chest exists and see it still functioning when I'm in the mining dimension. As soon as I leave the mining dimension (me being the only one there) it stops working 10 seconds later. As soon as my friend logs off or leaves the overworld, all chunk loading stops functioning in any dimension I'm not currently in.

Just to make sure it wasn't something strange with APTweaks I did remove them from my server and issue still seemed to happen.

It does appear that ftbchunks is properly setting the snbt files.
FTBChunks File - https://pastebin.com/YcVdzYY6
FTBChunks Party File - https://pastebin.com/EnfrgF5g
FTBChunks Player File - https://pastebin.com/VbWTkA3M
Note I'm only person in this party as I was messing around trying to get it working.

There has been one instance I have seen it work by chunk loading without creating the party. But there is certainly some combination of chunk loading functioning between taking multiple steps without being in a party, creating party, and relogging. I haven't been able to get it working again.

Seems like certainly an issue on FTBChunk's end so just going to link this:
FTBTeam/FTB-Mods-Issues#200

So, something I discovered is that the FTBChunks forceloading doesn't work right. However, the Minecraft command forceload works just fine. As admin of my server I used the forceload command to load a chunk, then I simply had an iron furnace smelt several stacks of sand into glass and I logged out right as it started. Waited about 10 or 15 mins just to be sure, logged back in and all of the stacks had smelted into glass. No one was one the server during that time.
If you control your own server this could be a work around until FTB fixes their mod.

On a whim I decided to try and use the previous version of FTB Chunks on my server, version 1801.3.3-build.113, and forced chunk loading seems to work fine with that version.