Transmutation Mana Reduction

This is not the set effect. I put on a partial set that gave 7 points into the mana reduction set effect and the effect did not activate despite saying I'd get "Lessen Mana Usage 3 Increase" in the talent window. Unfortunately I do not have any way of testing mana costs before I got this bonus, but I believe it's something that slightly affects mana cost at all times. For reference, with rank 1 Fire Mastery, one charge of Fireball seems to cost about 15 mp.

Are you sure it's not something else affecting your mana cost for fireball? The effect of the transmutation bonus should be mana_saving, but I hear it's glitched or something.

I have no idea what else would be affecting my mp cost. I made sure meditation was off and I did the casting during the day to minimize the effects of mp recovery when noting starting and ending mp. I was wearing nothing with set effects and all enchants were related to magic damage or crit. I was using a Casting Speed +40% -6% Damage Phoenix Fire Wand with no gem upgrades.

I have heard rumors that every 100 int contributes to a 1% mp reduction which would actually explain why a single charge of fireball, which would normally cost 20 mp with rank 1 Fire Mastery, is costing around 15 mp instead since I have 970 int with that wand on. Perhaps I should attempt to verify or debunk that rumor with the help of potion poisoning.

EDIT: Wait no it wouldn't... even if I round up, 10% of 20 is 2 mp and I should be using 18 mp per charge.

It is certainly bugged for mana reduction set and it may fix by a future patch.