Nami E Bug [GAMEPLAY]

I was playing a game of Nami support, with my friend playing as Ashe ADC, we were pushing bot lane. I used my E on ashe, she used one of the three stacks that my E gave her, so she had two left, I leveled up and put a point in my E, which made the remaining stacks on Ashe dissapear. I didn't try to re-do this in a private match and might have been a one time thing, not sure.