Jumping in to the party- RCS failed similarly 3 weeks ago after gOS update (not blaming anything/anyone). Many painful calls to carrier (VZ, PX8 Pro) after seeing conversations here and elsewhere related RCS function is now possibly carrier-controlled (if anywhere)- told to hard reset, which I am trying to avoid. Throughout those 3 weeks, RCS Chat settings object in GM consistently showed CONNECTED status after toggling RCS on/off and related reboots. This morning, another gOS update saw all of my RCS chats update fully to receive all comms from the 3-week interim. All conversations fully up to date, and I thought stars aligned and function restored. I tested by replying to a related conversation and-- RCS failed to send.
Completely reinstalled GM (from GPlay, current) for the first time since the issue started- RCS status shows 4006 Your device does not meet security requirements. Google Play Settings > About > Play Protect certification > Device is not certified. "Fix device issue" option >> "Couldn't fix device certification issue." Play Protect is enabled. An uncomfortable/excessive amount of permissions have been given to related google utils/apps today in troubleshooting >> no change.
All this said, I guess I just wanted to communicate (since I haven't seen anything similar in this thread in the last month) I had a window of about 10 minutes this morning immediately after the final components of the gOS update finished after reboot/on initial default profile logon (I believe during "optimizing apps" activity) whereby RCS function was seemingly restored/working. Shortly thereafter, it broke again. So perhaps some event/registration around the final stages of the gOS update actually kicked GM/RCS into a temporarily functional state. I of course don't know where to go from here.
Carrier Services was reinstalled as of today (was installed historically), but haven't cleared GPlay Services data. Don't quite understand the implications yet of related FPM (loss of) registration.