Ok I have maybe found the origin.
Steps:
Create a world in 1.16+ and go to caverns
Execute the command /datapack enable [non existent datapack] (like /datapack enable backpacks or /datapack enable pizza)
Wait 5 minutes in a gamemode where mobs can spawn
Search shulkers in spectator mode or another way
I repeat: Don't import datapack.
The world was created in 1.16 just after the release (not 1.16.1)
No, I don't use any mods. My Minecraft is fully vanilla
I can confirm this
Ok I have maybe found the origin.
Steps:
Create a world in 1.16+ and go to caverns
Execute the command /datapack enable [non existent datapack] (like /datapack enable backpacks or /datapack enable pizza)
Wait 5 minutes in a gamemode where mobs can spawn
Search shulkers in spectator mode or another way
I repeat: Don't import datapack.