Structures do not spawn mobs after a world is saved and reloaded

Opened an existing world in 1.21.10.22.  Switched to creative.  Traveled to my guardian farm.  Killed existing guardians using kill command.  Moved to AFK position but guardians failed to spawn.  Switched from hard mode to peaceful and back again.  Verified that Mob spawning is turned on. 

Step to Recreate

1) Download Derptopia Season 2 world at https://silentwisperer.com/5405-0 and open in 1.20.80

Alternate https://silentwisperer.com/downloads/

2) TP to 1413 61 -815

3) Kill existing mobs using kill @e[family=monster]

4) Move around platform and confirm that guardians are spawning back in.

Open the same world in 1.21.10.22.  Repeat the steps above.  Note that the guardians are no longer spawning. 

Expected behavior: Guardians should be spawning. This problem is being reported with other structure related mob spawns.