I've been working on a Guardian Grinder of my own design since shortly before the 1.14 update. And yes, sometimes when I first log in or arrive via nether in the area, no Guardians spawn in there for some reason. Logging out of the Realms server, waiting a few minutes for the sever to shut down and restart, and logging back in generally fixes the problem, it seems to be a bug.
Now, the fact that I haven't seen a single Squid spawn recently anywhere on the map...that bothers me a bit more. Finally got the Guardian Grinder working, and I can't get any more inksacs for making Dark Prismarine!
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I've built a guardian farm, one of those no-clear guardian farms, and it's not spawning any guardians!
I'm in a deep frozen ocean if that changes anything...
We'd need to know more about your farm design. Links, pics, video?
Guardians only spawn within the 58x58 confines of the monument, from floor level (Y38) to sea level (Y62). There is some vagueness in the spawning rules that suggest they might spawn above sea level, but I've never tried anything higher. They definitely won't spawn below Y38 or outside the 58x58 monument area. Spawning will also fail most of the time when the spawn area is exposed to sky (only water or transparent blocks above), so the spawn area must be covered with opaque blocks for good results.
What you want for any guardian farm is to have the desired spawn area covered, and everything outside of that either invalid (outside monument perimeter or dry), or open to sky (which reduces spawns by something like 95%).
Also remember that hostile mobs will not spawn within 24 blocks of a player, and will instantly despawn if the player is more than 128 block away.