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.
James Burkes
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.