Kayn orbs not filling the bar

So, this is an issue that has happened regularly for the past month and more.

Sometimes, upon leaving combat, orbs will not fill up the bar for some obscure reasons. There doesn't seem to be any logic on that, other than it always happens when fighting melee champions, so when I should receive red orbs, I just receive nothing.

This bugs occurs maybe on game out of 5, but is incredibly disruptive sometimes.