@Michael DiGregorio, Thanks for posting in Q&A. Based on my checking, the API you request cannot get primary user. But you can get the Primary User via the following API.
GET /beta/deviceManagement/managedDevices/{managedDeviceId}/users
Here is a link list the script to get device primary user for your reference.
Note: Non-Microsoft link, just for the reference.
For command "Get-MgDeviceManagemenManagedDevice", based on my testing, I have the same result when using "ManagedDeviceID" to query, the user principal name is not updated.
For this command issue, please open case to get help on this.
https://learn.microsoft.com/en-us/mem/get-support
Hope the above information can give you some help.
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