@Katie Parfinenko, Thanks for posting in Q&A. From your description, it seems the UserPrincipalNam is not correct or none after we change the primary user of one managed device. It affects some devices.
Based on my checking in my environment, I find it displays correct after I change primary user.
So I suggest open case to check in the background to see if there's any synced issue there.
https://video2.skills-academy.com/en-us/mem/get-support
Thanks for your understanding.
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