And yet, I don't suggest the initial label, of "phone", the device does during initial save.
It's also not selectable again, because it isn't in the list at all, after.
This is a vCard standard formatting issue.
As soon as I change the label to one that exists from the list, either while storing the contact initially or once RCS breaks, it immediately restores RCS.
For the two contacts this had been happening the moment I picked a label that actually existed in the list, RCS worked. When I deleted then re-added the contact with the default "phone" labels, RCS broke.
Not here to argue, sharing what worked and reliably. What's your suggestion?