I've had this happen to me quite a few times in which I am in a competitive queue while using a browser on my other monitor. Everything is normal through the ready check process, the game does the normal count down for preparing server and starting match, but at the end of the starting match timer, it doesn't bring you into the server and start loading the map. The queue timer completely cancels out, and you see "Return to Game" on the competitive gamemode card as if you just disconnected from the server. When you press the gamemode card to rejoin, it counts down from 3 with match starting again, and will again just cancel the queue once reaching 0.
I've fairly certain that the bug involves actively using a browser, and potentially playing a video in the browser, when the ready check reaches 10. I've also been loading into a lot of games recently that only 9/10 players load in, causing us to get booted out after the 2 minute timer ends. This leads me to believe I'm not the only one getting bugged out. Maybe the beta/early launch matchmaking bug has come back?