Guardians are not spawning in farm built in ocean monument at the correct level, even when all nearby mobs are cleared and caves are lit.
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I don't know what type is your farm, but guardians spawn condition of bedrock edition is different from (at least) Java edition.
In bedrock edition, it's necessary to build a floor for guardian spawning.
The floors of my farm are build by open fence gates.
And one more important condition is.
It seems to be changed spawn chunk range of mobs in the release 1.2.10.
In my short verification, the player stay over Y=31.
I don't known this change is intended or not.
Try this.