@Alex Demetriou Thanks for posting in our Q&A.
To avoid any misunderstanding, we appreciate your help to clarify something:
1.What did you mean the device X is no longer manageable in Endpoint? Is this device X is deleted from intune? Based on my understanding, if the device X is deleted from intune, we can't see it in intune portal.
2.If possible, please show us the screenshot about the device under Devices > All devices in intune portal.
Note: Please don't forget to erase private information
If there is anything update, feel free to let us know.
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