JUST happened to me a minute ago. I was mid-match playing Escalation on Lib Peak. Opened my scoreboard and then right-clicked to "expand" it and then boom: I was stuck there. The escape key did nothing. The TAB key (which is supposed to get you out of the expanded scoreboard) did nothing but change the scoreboard's bottom text as long as you held it, and once you let go, it reverts back to the expanded scoreboard text.
Very frustrating as I was having a good match and got locked out of playing. Sad that this issue has been occurring for at least a month.
Platform: PC/Steam.