We found a ocean monument and drained it several days after realm is updated to 1.12.0 to make a guardian farm, but no guardians spawned in it.

Then in order to locate the problem. we ask OP player to /locate the nearest monument, and it leads us to an area of open water with nothing on the sea floor, yet spawning guardians. It leads us to wonder whether if system determined area of ocean monument has been changed due to bug.