The vindicators seem to be confused to what hand to hold the axe... The attached screenshot shows this scenario.
Vindicators were spawned in via Spawn Egg.
The vindicator not holding the axe [left] seems to deal the same amount of damage even as the right vindicator [with the axe] even though the left is hitting with their hand.
The animation is a bug, them spawning left-handedly is not.