@Mike KT Thanks for posting in our Q&A.
To clarify this issue, we appreciate your help to collect some information:
1.Did you deploy an iOS store app?
2.Please check if there is any conditional access policy deployed to this user.
3.Please check if you can successfully sign in this app in an iOS device which is not enrolled to intune.
If there is anything update, feel free to let us know.
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
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