Vayne 1st Ult Tumble [Gameplay]

Whenever I play Vayne, I always cross a problem which can cause a death and turn the whole game around...It's whenever I use Vayne's Ultimate then tumble (Q) quickly.I keept trying to see if the problem continues by playing lots of customs and it did.The problem is whenever I tumble with her ult half of the time I DON'T go invisible...Please help me with this bug,it's made me loose lots of ranked fights already...</3