This has been occurring for many releases and can remember as far back as early 1.17.

New Mobs both passive and hostile mods of every type do not spawn. This occurs in all biomes that I have seen including underwater. Mobs that have been tamed, pre-existing like cows, horses, and chickens in a pen still exist and haven't de-spawned. Villagers as well still exist in houses that have been locked with Iron Doors (Enchantment Farming).

I have not encountered an Ocean Monument while this is occurring but will test if guardians have spawned under this condition.

The only solution to date that provides the longest solution is to copy the server files into the Server Directory again. The world files and config files are not altered during this re-copy. Essentially performing the same steps as a server update.