Fizz blocks Nami ult

Other day, played a game as Nami support, with a Fizz on enemy team, obviously. As they were attempting to take the base, I was standing near fountain as Fizz was just oddly standing in front of his team, between me and them, doing nothing. Just standing there waiting. I ulted to knock them all back from the turret and Fizz suddenly does his little Playful Trickster right in front of the beginning of the wave... which stopped/blocked/canceled the entire wave from continuing to his team. This particular Fizz seemed to maybe know exactly what he was doing/exploiting... like he was waiting for it or something... but I don't think that it's supposed to work that way... or at least it SHOULDN'T work that way. It was total bs, and entire team saw it happen and went 'wtf!?' because they were counting on the wave to engage.