It's supposed to work by downgrading everyone involved if any are not on RCS, because there is no other option. Which has been working fine for me at least, normal MMS issues aside (MMS delivery is often awful). RCS keeps an "is X using RCS?" list on their servers, and every attempt to message someone checks that (with a local cache)... and like >99% of those servers are Google, at this point, so it should be pretty consistent.

That said, I have no idea how often that fails in practice.

And that is how reactions are sent in SMS/MMS. Your app just isn't recognizing them to display them nicely. Maybe try a different one?