I run a server running 18w11a. I recently cleared an ocean monument, and built a guardian farm in it. Nothing spawned. After running some trials, I have determined it is an issue with 18w11a. An old world of mine also has a farm. Running it in 1.12, I got guardians to spawn, but not in 18w11a.
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Did you remove all the water? I have reproducibility across several worlds. In cases where the water had been removed and farms created, nothing is spawning.

I have this same issue. I made an Ocean Monument Guardian Farm on my realm in survival mode by completely clearing out the water. I then built the framework for the farm and then finally added a layer of water at the top so the rest of the farm is running water except for the top later. The farm worked great on the realm. I downloaded the backup for the realm and then played it as a single player using one of the 18w10 versions and no guardians would spawn. When 18w11a came out, I tried it again and no guardians would spawn.
I run this on Windows 10 using the Java version of Minecraft. I hope this information is complete enough to help reproduce the issue.
This is actually MC-125377.
Cannot reproduce. I went to an ocean monument, there were guardians. Ran
/kill
, the guardians were killed, and immediately, new ones spawned.