They literally feed us two separate traits at level 68 to make that possible, and it remains one of only 3 spells (with Despair) that gives AF3, so how can you say that's "clearly unintended"? If the devs wanted Flare to be purely a finisher move, they would have had it go the way of Despair long ago and
barred its use during Umbral Ice -- and then we would never have noticed that Flare is the weird
exception to nearly every phase-crossing nuance of our rotation.
Umbral Hearts are fundamentally designed to reduce Fire spell costs
during Astral Fire, which is why you don't lose any for popping Fire 3 during UI phase -- but Flare is the sole exception to that, consuming them even in Umbral Ice.
Aspect Mastery is designed to remove the cost of transitioning between phases, which lets us squeeze that little more damage out of AF phase in single-target -- but Flare is the sole exception to
that, too, continuing to have the same cost (even up to all of our MP) regardless.
If anything seems "clearly unintended," I would think it would be the lack of
consistency in our core mechanics, particularly the ones they've added each expansion.