Unknownreported this bug
I've been seeing an issue with model engine entities that have multiple oriented bounding boxes. I've tested this on a server that only had model engine installed, and was running the latest paper 1.21.10. I also tested on 1.21.11 build 100, and saw the same thing.
When hitting the entity, instead of only taking 1damage, every hitbox takes one damage, regardless of which one was actually hit. If an entity has 5 hitboxes, it takes 5 damage instead of 1.