Same thing here, when I got it in Round 4 and imbued it to Paint Battery, it allowed me to recharge almost up to full, but leaving it just shy of having one full charge, thus not allowing me to use it.
However, after using Refresher in Round 5, it gives me all the charges, and even shows that...
Had your same issue just today, two times in one match: first one Grey Talon didn't take any damage from Warden's Flask, and in the second one it seemed to not have any melee hitbox and again my Flask did nothing.
MatchID: 71426140
First occurrence at 6:34 when Grey Talon ults
Second...
