max.c.diener RCS requires Google Play Services at the moment. Mudita does not have Google Play Services and as such is incompatible with RCS. I highly recommend you turn off RCS and deregister from it so that your friend’s phones can learn that you don’t have this service anymore. It takes about 1-2 weeks for the deregistration to take effect. Just FYI.
This site lets you deregister from RCS if you forgot to when switching to MK (like I did) https://messages.google.com/disable-chat