I was doing some exploring in the latest snapshot and noticed that guardians were not spawning around the ocean monuments. The Elder Guardians did, however there were no regular ones.

Curious, I then teleported to a Pillager outpost to see the same issue. There were no pillagers spawning around it.

Regular mobs that spawn such as skeletons and zombies were fine, but is seems mobs that spawn around structures are broken.

Structures that have one time spawns, such as villages and end cities seem to be unaffected, it is only an issue with mobs that are meant to respawn around their structures.