I just wanted to comment and let you know that I was having the same issue and adding "runtime_identifier": "minecraft:iron_golem" to the behavior entity file fixed it for me. Animations also looks smoother. Thanks!
This is typically due to the transparent texture being saved in the wrong format. The texture pack creator would need to update the texture for this to be fixed. They should try saving their transparent textures in 16bcp RGBA format.
I just wanted to comment and let you know that I was having the same issue and adding "runtime_identifier": "minecraft:iron_golem" to the behavior entity file fixed it for me. Animations also looks smoother. Thanks!