i went into a creative world a spawned a wither then started spawning undead mobs around it and watched the undead aggro then repeatedly fail to hurt the wither

i tested zombies, baby zombies, drowned, baby drowned, zombie villagers, wither skeletons, skeletons, zoglins and zombified hoglins

i also summoned another wither with it in a small box and they could hurt each other

how to replicate

1. set difficulty to hard
2. spawn a wither
3. spawn any undead monster
4. wait for the wither to hit the other mobs with a random skull
5. watch them try to hurt it and fail