
IIRC that's a "health issue" and you should be able to suppress the notifications by selecting the corresponding item under Identities > Health issues. More details here: https://learn.microsoft.com/en-us/defender-for-identity/health-alerts
Just in case, you might also check rule exclusions as detailed for example here: https://learn.microsoft.com/en-us/defender-for-identity/exclusions#exclusions-by-detection-rule