Guardians are not spawning in preset "Floating Island"

This is affecting our server. I'm going to have to look at alternative options for the sake of getting drops from Guardians, as well as Witches and Pillagers. They are all related.

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.

After extensive testing I can confirm: Wandering Traders also do not spawn in the world.

Edit: Figures, after 2 months of not seeing a single one, one finally shows up for all of about 2 minutes before yeeting himself off a cliff into the void.

affects 19w46a

relates to:

MC-152081
MC-147659

Affects 1.15

Still affecting 1.15.1

Cannot reproduce in 1.16-pre2. Guardians spawning as expected in Floating Island/Deep Ocean.