Mobs that generate in structures, (I.e, Drowned in underwater ruins, or Piglins in bastions) generate invisible. Can be heard, can attack, can be damaged, though it is difficult as you can't see them. Quitting the game and rejoining the world fixes the issue, making the mobs visible again.

I haven't seen it affect villagers.

Seems to only affect mobs that are set to spawn when chunks containing structures are loaded in.