Undead mobs equips Illager banner instead of hold it in mainhand slot

Undead mobs picks up the item including banners in Normal or Hard difficulty. However they pick up and equip the illager banner as pillager captain instead of hold it in their mainhand.

 

1. Spawn some undead mob such as Husk above Normal difficulty.
2. Throw black or other colors banner to them.
3. Throw illager banner to them.