Structures generated prior to 20w21a (e.g. 1.15.2, 20w20b, etc.) are not considered valid structures within this version.
As such, mob spawns specific to certain structures (e.g. Guardians for Water Temples, Wither Skeletons for Nether Fortresses, etc.) no longer occur if the structure was generated from a previous version.
This is likely caused by the changing of structure names to full lowercase.