For something like texting/RCS, there are thousands of phones being tested thousands of time by thousands of people with multiple additional checkpoints (ODM, Google, Carriers). It's a core functionality so it's extremely tightly verified.
RCS was first implemented about three years ago for phones so it's shocking that this kind of bug happened now. I also worked on the initial infrastructure for this so I can't imagine how this bug was even introduced. My bet goes to it being a server-side issue, especially since it's carrier-specific, and not a software bug.