Tag not monitored by Microsoft.
Hi Mark,
Did you tried incognito browser to check if the account is not connected to another tenant? If you have tried basic steps I will suggest you to contact the support desk - https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
Hope this helps.
JS
==
Please Accept the answer if the information helped you. This will help us and others in the community as well.