It may well have LM, but the registration and unlock codes are missing from the xml. This is the same as what can happen if region 41 is corrupt or if it has been erased.

Do you have a backup of rgn41 made before this problem occurred? If so restore it and the codes should then be correctly generated in the xml and your LM subcription recognised.

The reason you can dl the map using the other unit's xml with UID changed is simply that the Garmin server needs to see that the GarminDevice.xml contains this info between </Id> and - <MassStorageMode>:

<RegistrationCode>XXXXXXX</RegistrationCode>-<Unlock><Code>XXXXXXXXXXXXXXXXXXXXXXXXX</Code></Unlock>-<Unlock><Code>XXXXXXXXXXXXXXXXXXXXXXXXX</Code></Unlock>