I have hollowed out a ocean monument to bedrock funneled all mobs from up to 8 block around the out side edge of the monument into a 4x4 square and the guardians float around at y31 y 32 area and will not fall any further. Even if released from the 4x4 cage they stay floating at the same level.
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-33641.
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