I built a guardian farm in 20w20b that worked fine, I updated to 20w21a and they stopped spawning. I started hosting the world around 20w12a, I later made a copy around 20w15a for singleplayer creative testing. after the 20w21a update both the server and singleplayer world will not spawn any guardians at the ocean monument. I copied the seed and created a new world and the gaurdians spawn in the new world.

Pictures attached:

The one ending in 17.21.png is the server gaurdian farm(no guardians spawning)

The one ending in 18.06.png is my singleplayer copy of the server(no guardians spawning)

The one ending in 22.24.png is my singleplayer that i created in 20w21a that guardians will  spawn in

If anyones wanting to test the seed is -5069835022302329143 and that ocean monument is at -1264 ~1247