I am sorry, llibrary wasnt the issue, it was custom mob spawner.

Ah alright, np. :)

Could you send a log from an instance where this occurs?

Instance? Sure... I use MultiMC too, I will see what I can dig up

I don't mean an instance as in a MultiMC instance. That was not very clear, apologies. :P
I more mean a session.

Oh yes, I did updated to the latest version, yet this is still occur

This is difficult, as the only log I know how to accesd is MultiMC's console log as my game is running

latest.log is created after a session is closed

And the bug also occur as I started my game, and persist, only when I removed llibrary that it stopped, so Im not sure about that instance, or session