As discussed offline regarding this issue, the only option is to check export cycles on AAD connector in AD connect operations tab and identify if delete was triggered on this group.
As per Azure AD audit logs, we can see logs for group add, but we are not able to see this group in delete audit logs. This indicates that this group was deleted more than a month back. Azure AD stores only 30 days audit logs by default.
This only option to check when this group was deleted in on-prem or moved to non-sync OU in on-prem is to check the audit logs against the group in on-premises. or you can also try to check audit logs on the OU where this group is part off.
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