[GAMEPLAY] Rek'Sai can't ult marked enemies

I don't know if this has been mentioned before, but I haven't seen it on here anywhere. I've been frequently experiencing a bug with Rek'Sai where I mark someone to ult, press R, and never actually ult them. I was recently in a situation where I was chasing a Jayce top lane who had very low health. I managed to hit him with my Q three times in a row, leaving him marked and within ult range for about fifteen seconds. I was pressing R repeatedly, but couldn't ult him, despite keeping him marked and staying in range the entire time.

I don't really know how to recreate this issue, as it pretty much just happens at random. Basically, when someone is marked and I go to ult them, sometimes it works, and sometimes it doesn't. I experience this bug at least once in every Rek'Sai game, often more than that. Unfortunately I don't have any videos of this occurring. Like I said, I didn't see this on here anywhere. Has this happened to anyone else?