IMO:
Currently, not all abilities/spells can be upgraded via Mastery points.
Whether intentional or not, that's just the current state of the game. It might be a bug, it might not be a bug.
I don't know if it's a public design goal that all abilities/spells will be upgradable via Mastery, at launch, for all classes.
I don't know if it's a public design goal that only some abilities/spells will be upgradable via Mastery, at launch, for all classes.
For those answers, you'll need to ask the development team, as only they know for sure.
You can ask them in the official discord. They might answer now, or at the next Q&A if you tag the question appropriately.