Tried this with just Mowzies Mobs and LLibrary installed (along with optifine), was able to reproduce the bug every time.
Using Mowzies Mobs 1.5.8 on 1.12.2

Test Setup (see screenshot)
https://i.imgur.com/NfZKDvZ.jpg

Server console log from reproduction of bug:
https://pastebin.com/NSGCmsar

Steps to reproduce:
[1] Build the test setup design (the outer redstone-slime pistons are just for keeping track of when the server freezes)
[2] Stand directly in the center of the water (water must be 2 blocks deep)
[3] Do /gamemode 0, float up and let all the foliaath rise out and snap at you all at the exact same time - Server/game stops ticking

Can you please send the server log?

Server log attached at pastebin link, but here's the log file as an attachment too
2022-06-15-3.log

Oh sorry, idk how I missed that

Try without MultiMC, please.

Same bug happens on non-MultiMC minecraft, though this time it actually unfroze after about 20 seconds of being frozen post-foliaath bite

here is the log
debug.log

Does the same thing happen on singleplayer?

Yes. This all has occurred on a listen server (singleplayer)

“Listen” server?

That's the term I've understood to be common for describing singleplayer servers that exist for Singleplayer clients to run on, such as playing Minecraft in singleplayer

Alright, thanks.

Tried testing this with a fresh Forge multiplayer standalone server - Same bug occurs. Server froze and eventually killed itself
debug server.txt
,