Ockie
Resident Lead Bender
On my unbranded Huawei phone using a MTN sim I believe it is still not using Googles universal RCS profile, and instead using MTN's implementation. I say this because I have the very same problem when trying to unregister as described in Ockie's original post on this thread.
My guess is then that it picks up the network identifier on the sim and then routes it via the network instead (if said network have implemented RCS). If not, then it goes via Google RCS servers.