This keeps happening for me on a Win11 Asus Rog Ally, but only for Brawl Matches. The game will still run as usual, the sounds will also imply that a match has in fact started, but the "cancel queue" button suddenly disappears (and the "friends online" button will turn into a "settings" button). It is not possible to leave the queue screen or cancel the match (which I cannot join). The only option I have is to end the game via steam and restart it a couple minutes later (as not to get flung back into the very same queue)

Interestingly it only seems to affect Brawl games (open queue or matches with a friend). I can play Pioneer and Standard just fine. Reinstalling the game did not help.

Same issue by my side.
Game freezes for around 20 seconds (in the slot in which the game should boot up) and thyen unfreezes saying "starting match" on the bottom... of course it will never log into the game since it is already skipped.
Today after a day of PC sleep, a 1.5 GB patch happened on my game, still nothing

This is a screenshot. Can't capture log.