To add to this (I feel a change should be made to this ticket to reflect all of these):
Guardians do not spawn in the Monuments.
Witches do not spawn in Witch Huts (Except for the initial witch)
Pillager Outposts will not spawn any Pillagers
No Village stray cats will spawn. The only way to obtain a cat is the first cat spawn in a witch hut. I also attempted to get them to spawn in The End and Nether. Nothing.
So, we have a TON of farms broken with Buffet mode.
Cannot reproduce in 1.16-pre2. Guardians spawning as expected in Floating Island/Deep Ocean.