Okay so I was gonna use the mob bag to move my mending villager out of a swamp, after I finally got him in the bag, I placed him down as a test and poof! Nothing but a puff of smoke and an empty bag! I tested this on like, 3 other villagers and it does the same, I also tried on a sheep and a pig and they also disappears.

I got footage of it happening but it didn't upload for some reason so here's a link to an unlisted video of it:
https://youtu.be/LuKUP_PLad0

This is probably important to fix.

I took a look at the video, I suspect something very odd is going on here. The texture updates, but the mob bag's tooltip doesn't update meaning the whole capture process didn't complete

What version of the mod are you running, and could you send me a list of mods/datapacks you're using?

Running the latest version, 1.9, the mods I have are attached below in the modrinth pack <3

One to Keep Forever More 1.0.0.zip

That's perfect! I'll see if I can replicate this in your pack

This certainly doesn't happen in vanilla, so I'm suspecting some kind of conflict

Should also mention I have the villager trade rebalance on

Update on this, I think this is most definitely a mod incompatibility! But this is not limited just to the mob bag, teleporting ANY entity with a command seems to just kill it?

I'm getting this sound playing when teleporting an entity:

Will try to narrow this down to an individual mod

Found it, disabling Trailier Tales stops the issue, I would suggest raising a bug on their github

I can also go ahead and do it if you want

Yes please, I'd appreciate it, they're more likely to listen to you.

Opened issue here: FrozenBlock/TrailierTales#16