Mana regen bug?

Hi dear GGG, recently i have problem with a mana sustain. Curently i have 4706 mana with 925 mana regeneration per second, my spark cost - 233 mana per use, i cast 3.56 times per second which lead us to total 829 mana per second mana cost.
If i'm not wrong i should have 96 mana regen in reserv, but right now my mana gets low when i spam spark, which shouldn't be a case.

At the same time, when i switch gear to less mana and mana regen, this promblem just disappears. With this gear i have: 4408 mana, 732 mana regen, mana cost of spark - 224, cast per second - 3,23, which lead us to total 723 mana cost per second, which leave me with just 9 mana regen in reserv, BUT THIS TIME I'M NOT GET LOW AT MANA AT ALL.

Is this a bug, or i'm just don't understand how it should work?

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