Guardians do not spawn in ocean temple chunk

After updating the world to 20w21a, guardians no longer spawn in my guardian farm, which spans the entire ocean temple chunk(s).

 

Note: my world is multiplayer, but no mobcaps have stopped my farm

 

I tried to replicate the glitch on new worlds but with no success.

 

If I create a world in the 20w21a, guardians do spawn in an untouched temple.

If I create a world in a previous snapshot and update it, guardians still spawn.

If I destroy the temple, kill the elder guardians, and place a layer of soul sand (all guardian farms use soul sand) guardians still spawn.

 

It appears this issue occurs only on mature worlds, or on multiplayer.