To Reproduce: try to leave a match when the black screen starts showing. You will get stuck in an infinity loop with the transition music still playing forever, and the Overwatch symbol in the bottom right as if the match is loading, but it never does. Ways to leave it the loop include opening a loot box if you have one, or restart the game.
u can also say “Leave Match” and u might stay in your group. Happens often too me XD