In the clip you can see that both Ezreal and Karma have finished being airborne before Yasuo ults. I don't play yasuo, so I don't really have the means to replicate the bug, but it's clear here that his ultimate is improperly activating, knocking up Ezreal and Karma a second time, rather than keeping them airborne as his ult is supposed to do.
They still stood still so im guessing they were technically still airborne, the animation was just finished. If from the timeframe they are still airborne he can ult them eventhough the animation might look weird. Idk how long yasuos q knockup is so i cant say for sure. The ult always knocks the targets back up during its duration.
Did it look like that in game too, or just the replay tool? I've seen some visually weird stuff happen in the replay tool. I'm actually impressed he was able to ult considering he was facing away from them, most times his ult bugs out and won't activate unless he's facing the target.
