[GAMEPLAY] xeraths passive not working

it did not happen any time early game, but later(about 20-30 min onwards. the game lasted almost an hour) game i would start noticing that even though i had no mana, and the passive was "active"(i.e. done with the cooldown), i would not get the mana restored upon basic attacks. this stopped happening after a minute or two, but came back a couple times. i tried to experiment to see what caused it, but found nothong. once, it started working again upon dealing the killing blow to a minion, but i tried it another time the bug happened, and this did not stop the bug.