[Gameplay]Fiddlestick E doesn't always bounce when used on blackshielded target

This have happened numerous times, the enemy fiddlestick used his E on me or my ally and I immediately used black shield on his target after he uses his E, but sometimes his E will still bounce off while some other times it just disappears. I am very sure it is not because of distance because everytime I use blackshield we are in a grouped siege position and we stand near each other. Not sure if it should disappear or if it should bounce off even when the target has blackshield, but the fact that different outcomes resulted in the same game it must have been a bug.