@Jenkins, Brian David , Based on my check on the iOS device in my Intune environment. I find the UDID is also empty. I think this can be the reason why we get blank or null from Graph API. Please go to Intune portal to double confirm on the device.
Then I go to do some research with UDID. It seems to be a unique identifier that Apple uses to associate a device to an iOS developer account. We can check if there's UDID on Apple device. if not, try to contact Apple support to help generate the UDID and then re-enroll into Intune to see if we can get the information.
Hope it can help.
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