2 seconds longer ult
Yeah it would be nice if it was so organized but really you get many programmers working on the same project. A small company might have 20 to work on console to console ports or full handheld games. For a game like overwatch you could easily have hundreds of programmers touch the code. And senior programmers likely work on the project durring inital development and then less so afterward. You allso will have 1000s of files with lines and lines of code in each. Its easy for a new guy to see "make genjis ult stay active even after being stunned" in the workload/bug database and put in some code that looks like it works. But he didnt do his homework. And when he tests it he could easily fail to notice the extra swings because he would have to check the hp of the enemies. Even the in house testers would be likely to miss it.
I think it's probably when they reduced the ult drain time to 0, it pulls the blade out immediately in the code but the animation hasn't caught up, hence extra time. Not totally a bug, but something they'll probably fix.
It brings up the question though, how do you deal with the fact that Genji is extremely vulnerable while pulling out the blade and now he doesn't have the added bonus of getting his ult back.
Risk/Reward ratio, if you get outplayed while using it you don't get it back. I think that's the correct way to handle it regardless of how long the activation is.
That being said they could reduce the activation time to make it less punishing, but no more freebies when getting outplayed now.
Edit - Geoff confirmed it was a bug:
https://us.battle.net/forums/en/overwatch/topic/20759317874?page=3#post-56