Spawned in Guardians on land with a mob egg. Damaged them with instant damage II/Instant Health II potions. Some of them freeze in place. When they are frozen you can hit them 1 more time before they become invincible. Some freeze, but are able to be killed.

/kill @e[type=minecraft:guardian] returns 'No entity was found'

While looking at them with the F3 screen open, the show up as targeted Entity

Leaving and reloading the world unfreezes them as if nothing happened.

Also A GIF : https://gfycat.com/PalatableDeepHarborseal

--------------

Bug first found with guardians and potions.I have not confirmed other types of damage, but I have found that the same happens with drowned. Will continue to update. Behavior with commands, invincibility, F3 screen and leaving and loading the world is the same.

Bugged Mobs:

• Guardian
• Drowned
• Evoker
• Zombie Villager
• Husk
• Skeleton
• Creeper
• Turtles
  *