I have a clip of this happening here:
You can see that the rogue pops sprint (gains the buff on the target frame), then I go halfway through a poly cast and then cast nova and he IMMUNES it. This is not how imp sprint is supposed to work.
Improved sprint 2/2:
“Gives a 100% chance to remove all movement impairing effects when you activate your Sprint ability.”
It says nothing about making you immune to movement impairing effects. Especially over a second after the ability has been used.
EDIT: btw it helps if you playback at like 50% speed