If you have 30 red AP and then cast Fireball, you don’t actually get the 2 AP gain from destroyed tiles.
It looks like the AP gain is calculated before the AP spend, therefore, it thinks you’re already at max AP even though you just spent 8 red.
Same thing happens with Hex Bolt and Lightning Storm I believe. If 2* Storm casts her green with 30 AP you will have 18 no matter what (even after taking out nearly half the board).
Looks like they need to apply the effect before the cost so that abilities like Lightning Storm and Blammo! destroy the correct number of tiles. This has the side effect of not giving you the AP since you’re still at the cap when the AP generation takes place.