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