Strongholds will often spawn with floating glow lichen inside them.
Steps to reproduce:
Generate a new world in 21w08a (Creative mode / Cheats on)
Set gamemode to spectator
locate and go to a stronghold
look around. - Roughly 50% of strongholds I saw had this issue.
Here are some confirmed places where this issue occurs:
Seed:
-3956006796778357867
Coords:
593, 2, 5296
-976, 11, 1482
-4605, 2, 3248
-4304, -39, -2054
Duplicate of MC-212146