Hello @Stiv Ostenberg ,
Thanks for reaching out.
I would recommend you to ask the experts in the dedicated Community forum over here since Microsoft Account is currently not supported in this Q&A forums, the supported products are listed over here https://learn.microsoft.com/en-us/answers/products (more to be added later on).
Meanwhile, here are my thoughts on this issue, I guess this issue caused due to same username '******@stiv.com' used with MSA (Microsoft Account) as well for Work account (Office 365).
Could you please confirm, is that true in your scenario? also did you get "Pick an account" similar to below when you access Xbox ? if so then did you pick right MSA live account for sign-in instead of work account?
also do you remember of using any other account alias for Microsoft account in past. Its worth to try accessing Xbox by using user account with suffix of outlook.com like 'stiv@harsh.com .com' because outlook.com is default alias by default until you had changed in past. If non of these help you then I recommend you to reaching out dedicate team mentioned in above link.
Example from my MSA account alias setting:
Hope this helps.
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