If Brand’s abilities instantly kill a unit before Blaze is applied, he doesn’t get mana back

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It is pure a unit kill that is ablazed. They don't want you quickly insta-killing waves with W with minimal mana cost (because of mana refunds). The mechanism is to help you get some more mana back especially early game, not completely negate your mana costs.

You can't just 1 shot a whole wave with W until you have tons of AP at which point I don't see the issue with the mana sustain.

The annoying part of this is early when you last hit 2-3 minions which are already low with W and you won't get any mana back.

Vel'Koz can last hit 2 minions with Q and refund 100% of the mana cost, or he can last hit 3 minions and get back 150% of the mana cost. Brand is getting the short end of the stick.