Entities currently don't use the boundaries of the models they hold/wear for their visibility checks. This means that when they use larger models, depening on what angle you look at the entity, the model might disappear even while it is displayed on the screen.

It basically happens when you face away from the entity that has the given model in his hand/worn as helmet, but.. the model is facing the other way and thus still in your sight. In this scenario the client considers the entity to be "out of view", even while it's model stil actually is in the view.

An example that shows this bug
https://youtu.be/Y5RMxt7VQSo

If entities were to include the model boundaries in the visibility checks, this bug wouldn't occur (assuming that using such large models isn't considered "abuse", as Minecraft explicitly allows models up to 3x3x3 dimensions large if I recall correctly)