I and 4 other players (1 from my team and 3 from enemy team) got disconnected at the same time in a competitive game in the middle of the first round.
I returned to the game before it was over and played until the end, but I still got -50SR and 15 minute suspension, which was ticking down even while I was playing, as by the time the game finished, I only had 6 minutes of suspension left.
Is this normal? To return to the game, but still get punished?
Even though it was some Blizz server fault?
Yes it’s normal. Their automated system can’t tell the difference, even if you return to the match. Might be a bug, but it’s happened to me a couple times now; Come back and help win the match, SR loss and suspension. Yay
Wow… I mean, I understand getting that punishment if you don’t return to the game at all, or at least there should be a time limit to return, during which you won’t get punished, otherwise people coul grief others, but the current system seems pretty crappy.
Not to mention the leaving very early system, where you don’t even have a chance to return, game is canceled, you’re punished and that’s it, can’t do anything about it 
Players must return to a match within 2 minutes, after that time, the game considers it abandonment and grants your allies the option to leave without penalty. Even if you return after two minutes, it is still a marked penalty with the appropriate suspension. More details about this policy can be found here:
If you are experiencing disconnections, please avoid Competitive Play entirely until you can troubleshoot and correct the issue. Follow these steps or visit the technical support forum for assistance: