Tirisfal Regalia(Mage) Two-Piece bonus

[Euthanized]

The two-piece bonus of this set states that it "Increases the damage and mana cost of Arcane Blast by 20%", which rings true. But sometimes AB acts as if you never had the set bonus, reverting to the "default" version. Normally it costs 195 mana to cast it(disregarding its buff). With the bonus, it costs 234 mana.

As I said, it sometimes reverts from the (2) Set version to it's default one, in doing less damage and costing less mana than it should. I've noticed this occurred whenever I got the Arcane Blast buff.
I'm not entirely sure if the buff somehow overrides the Set Bonus, if this is a bug with the set bonus and/or Arcane Blast Buff, or if it's working as intended.

Any feedback of this would be great!

[Imperium]

wondering if it's something to do with other talents/spells that also increase AB by the same amount?
because shaman T6 4pc bonus will not stack with Ashtongue Talisman or Stonebreaker's totem
the enhance T6 set btw. they overlap or don't proc at all, so i ahve to choose one or the other

wondering if its the same case.

[Euthanized]

Well the only other thing that directly affects Arcane Blast is the talent Arcane Impact, which "Increases the critical strike chance of Arcane Explosion and Arcane Blast spells by an additional 6%". It doesn't seem like those should overlap one another, on the count that the T5 2-piece bonus just increases AB's damage and mana cost by 20%, not crit chance.

[Imperium]

you've a point there. what about any additional trinkets or spellbuffs that increase your damage by 20%?

[Euthanized]

So far it's only Arcane Instability that directly increases spell damage(Also critical strike chance) by 3%.
There's Mind Mastery, which stems off of my intellect, so it might not be that. Arcane Power makes me spells cost 30% mana while giving them a 30% more damage output, but when I tested that and the Set Bonus, they seemed to be working together(When the Bonus decided to actually work). So I'm not entirely sure. It could be one or the other.