This is a completely random event usually occurring on a server but a captured mib turns into a pig. It seems to happen at the initial capture event and not from being stored either in a backpack or other inventory etc. It happens with all kinds of mobs including ones that can be traded with eg villagers piglins etc.

Minecraft version 1.19.2
Latest version of mod

I just had this issue happen to a villager that was in a boat when I was catching it
It hasn't happened on anything else on the 1.18.2 version so far

Cannot reproduce

I'm running into this on 1.18.2, though I've noticed it happens relatively consistently when the mob is in a Create seat.

I have this issue, too. Also running on 1.18.2.

Edit: It seems like it always happens to chickens...