They will use the iMessage protocol if supported by all clients. If not, they fall back to the next best thing supported by all clients whether RCS or SMS/MMS. In your case (possibly before iPhones supported RCS) the "next best thing" was apparently SMS/MMS.
This is the correct behavior.
I think you're also falling into the common trap of automatically thinking whatever Android supports is like, the correct and open standard.
In reality, RCS's history was an absolute mess of incompatible implementations, pushed and owned by some of by Apple's direct competitors. It's really not any more the "correct" standard than iMessage is and it does not support E2EE outside of Google's proprietary implementation. https://en.wikipedia.org/wiki/Rich_Communication_Services#De...