Before 1.13, I drained an ocean monument and set up a guardian farm (see screenshot). Now, it's completely useless, as it acts as a regular ocean where only regular hostile mobs spawn. No guardians at all.
It's quite easy to reproduce the issue, just find a 1.12 seed with an ocean monument and update to 1.13.
Environment
Windows 10
Java 1.8.0_51 64 Bit
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I have the same problem. My Guardian farm used to work in 1.12 but it has stopped spawning any Guardians after upgrading to 1.13.
Please check if that's still an issue for you in the latest 1.14.4 stable release or the latest 1.15 development snapshot.
Was the world open in snapshots?