While the 2.1 text says "MAY" it doesn't specifically say that it overrides 2822, so it's not 100% clear. It's unclear what the order of precedence is in interpreting 2822 vs. 2.1 vs. the presumably agreed-upon obs-phrase text, etc.
So MUST means MAY except where (e.g. obs-phrase) MAY means MUST. Unless of course MUST means MAY again.