When I damage Plague Doctors they eat to heal. They also pick up nearby items. When they are invisible during one can sometimes see the item they picked up.
I noticed that when I farmed plague doctors with a spawner (I use the enchanced spawner mod which allows me to obtain spawners with silk touch and spawneggs very rarely when killing a mob) when a plague doctors eats any item they are holding which was previouly picked up that the game crashes.
The crash happens usally when a plague doctor picks up a item that can't normally be eaten and then tries to eat it. In my farm one of these items that they pick up is the plague leech. They pick it up, I damage them and then they try to eat it to heal and the game then crashes.

I couldn't figure out how to put in lables so:
Version is 1.20.1 of Minecraft
Mod version is "Rats 8.0.6"

Crash report -> https://pastebin.com/jYvrUg4R