This is not as much a bug as it is some kind of design flaw, but it is something that needs fixing nonetheless.
When Zyra dies and her passive activates, it registers input before it can actually fire. As an example, a while ago I played Zyra, and at some point i was ganked mid by 3 people. I decide I can't run and turn around to face them. I use my non-ulti skills and catch two of them, getting them kinda low. Then as a last act, just before I die, I spam my R to get down my ult. It goes down, and the two I got low are knocked up in perfect alignment for my passive and what tould have been a double kill. But when my passive is actually ready it has already decided what direction to shoot from when I spammed my R as I died.
I hope you can understand how incredibly frustration this is, and it should be fixable.
In short: Zyra's passive takes input before it can be activated, which leads to misfires regularly.