I just found this in the release notes:
173422 Symptom: The Dashboard page of a Mobility Master incorrectly displays the status of
some APs as Down although the APs are up.
Scenario: This issue occurs when all the SSIDs in an AP group are disabled. This issue is observed in a Mobility Master running ArubaOS 8.2.0.1
i can't see the bug ID in 8.2.1.0. maybe it's fixed in the latest firmware version 8.2.1.0.