Ocean Monument only contains Elder Guardians, regular Guardians hadn't spawned when the Monument was generated and didn't spawn while exploring it.
Game is locked to normal difficulty, no other mob spawning issues seem to be happening. Only Guardians are affected. All other mobs are spawning normally.
Thank you for your report!
We're actually already tracking this issue at MC-146919, so we will resolve and link this ticket as a duplicate.
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