There may be some data left in Chamberlain/MyQ database that is causing this issue. If you are getting a specific error stating that it is still registered that would be pretty reliable as a cause.
Have you contacted Chamberlain support to see if they can ensure any and all data regarding that unit’s serial is removed?
We may be able to provide additional assistance if you provide the Serial number for the hub via secure message. You can find the secure message tab in your Surety System Manager here.
Based on the information submitted, it would appear that the hub itself was not removed from the associated MyQ account, just the garage door.
In order to proceed, you will need to delete the hub from the native MyQ app. To do so, log into your MyQ account and tap the menu bars on the upper left corner then tap Device Management and lastly tap the hub and press the trash can icon associated with the hub itself. The hub itself will need to be removed prior to it being associated with your Alarm.com account.
After following the above steps, are you able to remove the hub from the native MyQ app and re add it?