It happens when I am using addons to change a wither skeleton into a village guard. so the entity has the family 'villager' and the shell of a wither skeleton. The entity is tameable, and whenever there is a fight between two tamed , or untamed and tamed withers the server crashes after about 5-6 hits. The timing of the crash seems to be pretty exactly correlated with the amount of damage that is dealt in either direction. It seems to be 5-6 hits either way and the game crashes.

I have attached the world that I am using. It is stripped down to just the village guard addon in a flat world. It still happens faithfully whenever I run it on the Realm, but if I run it locally there is no issue with the withers fighting. The fight goes to completion with one of them dying and no crashing.