when mobs like zombies take a shield and equip it in the right hand (second hand) they have their arms down like a player
and when these monsters see you they do the same animation as using a bow
this bug happens to any undead mob that can use armor and tools here is the list:

• Zombies
• Husks
• Drowneds
• Zombie Villagers
• Zombie Piglins
• Skeletons
• Wither Skeletons
• Strays
  how to replicate:
  drop a shield to any of the mobs in the list above if it catches the bug it will happen immediately