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This does not happen often but sometimes when a mob with a model spawns, the death animation will instantly play and from that point on, the model is stuck on the last frame of the death animation. It'll still walk around and attack (if it's hostile). Basically, acting like a normal mob. The only thing I can think of that may be causing it is setting the mob's max health and health:

mob.getAttribute(Attribute.GENERIC_MAX_HEALTH).setBaseValue(30D);
mob.setHealth(30D);

But it's too random and out of nowhere to really understand what is going on.