[GAMEPLAY] Mana flow band not working properly

I searched for this issue and found some other posts about this rune having issues from a couple of months back and it seems it still hasn't been fixed.

In this play you can see the rune getting activated when I start to recall (yes, I was playing Ahri) so I decide to ult into Zilean face because the rune would give me some mana back. The ultimate didn't indeed spend any mana but didn't recovered the %8 missing mana. You can see that I ulted (mana doesn't change) and used W so the bar got from 112 mana to 53 (W mana cost is 50, plus 1 from passive mana regen). I should have recovered ~40 mana but didn't get it, leaving me defeseless as I was going to cast Charm into Zilean at point black.