As you can expect with pretty much any new system, Overwatch's Competitive Play turned out to be a rather glorious mess. If you're interested in the details, I've done on a full rundown on all of the issues and how to fix them, but to put it simply, the system hates seeing disconnects so much its willing to practically negate the entire match, no matter who disconnected, or even if they came back within a few seconds.

The end result of these issues is a ranking system that might statistically be perfectly fair, but in reality feels like an absolute slog to go through. You can win three games in a row and barely gain any rank, then lose one single game with a leaver on your team and find yourself going down an entire rank because the system switched into "punishment mode". If you've encountered issues like this, you'll be glad to hear that Blizzard is aiming to have them corrected at some point this week, as noted by Jeff Kaplan:

LeftyChev: "There are a ton of threads on this issue now but I haven't seen a blue post addressing it yet. People are experiencing a disconnect from a game and then quickly reconnecting. Their team wins and they either get a loss in placement matches or they loose skill rating.

Is this a bug or is it working as intended?"

Jeff Kaplan: "We have some fixes coming (hopefully) next week."

As you can clearly see, Kaplan didn't specify which issues the team is currently working on, but the fact that they are all interconnected leads me to believe its going to all of them, or at least the majority. They've already managed to fix Control maps in only a couple of days, so hopefully these problems will get the same sort of quick treatment as they really are obnoxious to encounter in the wild.

Until the fixes do appear, however, I would suggest you don't join Competitive Play with even a slightly unstable connection as a single second without communication to Blizzard servers will result in you getting a loss and a massive penalty to your competitive rating, even if you reconnect and lead your team to victory!