Long story short, I played Vel'koz thrice and found this bug happening more than once in 3 separate games.
It's one of those random bugs, I couldn't find the trigger but this is what I saw:
1. Use Vel'koz Ultimate.
2. Vel'koz's animation is doing his ultimate laser mode (his animation is fine).
3. No laser is shown on screen (there is no laser firing at that time).
4. Damage is done onto target (Only the very first hit).
5. Vel'koz ultimate animation ends automatically.
6. Vel'koz is movable again and ultimate is cancelled.
It happened in all 3 DoomBots games of 2bombs and 5bombs difficulty.
Since PBE Server is down at the moment, I can't further investigate. Please, if anyone is reading this, do a follow-up if possible. I might not be available for more tests in the next few days. Thanks.