It seems like if you execute the combo too fast or if you Q an enemy right before doing the combo, attempting to do the R-ally + W combo will cancel your W and the ability will get on cd.

Maybe there is a link between the fact you Q just before using the combo (So the game may understand that you are doing the Q-R combo on enemy so he blocks your W).

I have recorded the bug, just tell me how I can send you the video.