Setting up and managing device configurations using Intune
@Anon Account Thanks for your update. From the information you provided, the iPhone is an ADE enrollment device.
For ADE enrollment device, if the IT configure the setting "Locked enrollment" to "Yes" in the Apple enrollment profile, we can't remove MDM management profile manually in the iPhone.
https://learn.microsoft.com/en-us/mem/intune/enrollment/device-enrollment-program-enroll-ios#create-an-apple-enrollment-profile
If we want to we want to completely remove an ADE device from management by Intune, it is needed to unsigned the device in Apple Business Manager.
https://learn.microsoft.com/en-us/mem/intune/remote-actions/devices-wipe#retire-an-apple-ade-device-from-intune
Hope it will help.