Apps Admin Center - Inventory - Updating channel

With running the (unsupported!) command line you "just" instructed the device to perform a channel change the next time it runs an update pass. So you could trigger the Office Automatic Updates 2.0 task or open an app and go to file > account > update now to perform an update (and the actual transition to the Monthly Enterprise Channel). Only after that the MEC channel will be reported in the inventory and Servicing Profile would deem the device as "in scope" and keep it updated.