I updated my survival world to snapshot 20w21a. I went to my guardian farm and it didn't seem to spawn any guardians. After trying to see the problem, it looks like the game doesn't recognise the Ocean Monument that I had modified as a structure at all, so it doesn't spawn guardians and doesn't show up when I use the /locate command. This happens to any structure I have ever visited, like Nether Fortresses which don't spawn mobs at all, Witch Huts or thatPillager Outposts that also don't. Structures where I had never been seem to work as usual.