[Gameplay] Kayle Q doesn't refund mana on champion hits through the minion wave

So I played over 40 games with kayle support this season and I've noticed that her Q should refund mana on champion hits. And it does, if it hits champions only. If it hits them through the minion wave, it refunds mana quite inconsistently - see the attached vid.

First I use Q through the wave, and it doesn't refund anything. Then, I use it on 2 dummies, and it refunds 100% of the cost. I then spam it through the wave, and at first it doesn't refund anything, but then, when either minions are not at full HP, or my mana is not full - it refunds 99% of the cost - you can notice my mana goes down by 1 each time i cast it.

I see that as a bug. Thanks in advance