Happens to me on servers ever since 1.8. It seems to create some sort of phantom item in the slot but when you go into your inventory and click the spot, the item suddenly reappears
Maybe the wither is currently searching for other entities? Kinda how it likes to attack literally EVERYTHING before it starts chasing the player. Just my opinion. It could also be targeting one of its own heads to chase.
I can confirm the latest version, I was trying to have them attack at night, but the trader just drinks an invisibility potion. Maybe the llamas don’t attack when the trader is invisible as to not draw unnecessary attention to the trader’s location?