And you're right, a vendor could incorporate novel patent-encumbered instructions into their otherwise RISC-V (based) CPU designs. In which case perfect compatibility wouldn't be possible.

Patterson, I believe, has said that folks like Intel intentionally do this, add rather pointless new patented instructions so that they are always "embracing and extending" (my words) their own designs to prevent true compatibility; and thus cloning by others. And that that's one big reason why it was worth creating RISC-V.