@Woods, Alfred Thanks for posting in our Q&A.
It seems that there is an app protection policy about this Outlook app deployed. Intune app protection policies limit users to one managed work or school account per app.
https://learn.microsoft.com/en-us/mem/intune/fundamentals/end-user-mam-apps-ios#manage-user-accounts-on-the-device
Hope it will help.
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