The Abyssal Dispersion node is interacting oddly with Abyssal Echoes’ Mana Cost. Depending on the formula, 5 points in Abyssal Dispersion should reduce the cost to:
(35 + 15) x 0.6 = 30
or
(35 x 0.6) + 15 = 36
Instead it’s displaying as 32 or, sometimes, 27. I don’t have any buffs that could cause additional reduction in cost and haven’t had any when I checked.