@Stephan Bisschop , From your description, it seems two users can't get any device configuration policy. If there's any misunderstanding, feel free to let us know.
For the two affected user, try to login one affected AAD user on one affected device, go to Settings->Accounts->Access work or school, find the account, info and then click sync to see if it can sync successfully.
On the device side, check the DeviceManagement-Enterprise-Diagnostic-Provider Event log to see if there's any error related.
Please check the above information and if there's any update, feel free to let us know.
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