New-Rengar bug (kind of)

So was palying a rengar game and i noticed i didnt get stack when i killed the enemy kayle so i had to go top again. i did get it later one but thats out of the point. i didnt get it the first time under very specific circumastences. I come from behind her while she is hitting our karthus in the top lane. i slice her and scratch her alot but she kills him. she got hit by a couple of tower shots for that but when she had around 100 hit-point she utled. i couldnt damage her cause of that and then she flashed. the tower got me the kill still cause i damaged her earlier but at the last 1.5 seconds i hadnt damaged her cause first she ulted and then flashed while a tower shot was following (cause im silver). i didnt get the stack cause i couldnt do damage throught the ultimate and even though i did get the kill i got no stackz. i think riot should make it like "you get a stack of bonetooth neclace if you either damaged the target that got killed in the last 1.5 seconds or got the kill on them". would solve very situational cases like this.