I had a pocket do this in a game a few days ago as well, matchid: 35202959, around 13 minutes in. Same outcome, by the time we figured it out and killed him we were at too much of a disadvantage to win.
I tested a bunch of other heroes and it seems like it only affects M&K. It does work for me once in this clip but I think it's an interaction with the golden statue allowing it to happen.
I've been running into this a lot lately and finally narrowed down a way to reproduce this. Basically, after performing a heavy melee cancel in air, there is an already reported bug where your FOV will decrease for a bit. If you wait until the FOV changes and then slide again, your FOV does this...