Guardians not spawning

Guardians are not spawning in old monuments, but are spawning in newly generated monuments. Not sure exactly when the old one was generated, but it I know it was an early 1.16 snapshot and guardians were spawning there at one point. The new one was generated in 1.16 pre-release 1.

This seems to be a separate issue from the "monument" vs. "Monument" problem.