Competitive: match suddenly ends without warning

I was in the middle of a competitive match, when all of a sudden, around four people left at the same time. Not sure if they were on my team or not, but I assume those were on the enemy team since they were losing badly. A few seconds after the disconnects, I'm suddenly back on the main menu myself, with no explanation as to what just happened. No message, no warning.

At first, I thought I disconnected, but I wasn't logged out (I still had internet) and the "Play" button didn't say "Rejoin", so I assume the whole competitive match was just... finished? Usually, when the other team leaves, you still go through the usual endgame stuff: the victory screen, POTG and endgame cards. This time however, I was just sent straight to the main menu.

I checked to see if I was maybe penalized or something, but everything seems fine. Unfortunately, I don't know if it registered a win, but it seems like I got points instead of losing them. Or maybe nothing happened? I have no idea if that match counted as a win or not because of its abrupt ending.

What happened here?

Take this with a grain of salt, but what you describe sounds very much like one or more players on the enemy team have discovered a bug in the game that allows them to nullify the match and possibly not acquire a loss.

I've had competitive games where the enemy team had only one player on their side, so it wasn't a game-design feature that happened to you.

I'd check with the battle.net forums and report the issue there.