This has been a problem for me for over a year. I can play all of the other matches with no issues. When it comes to Brawl (including Standard Brawl), stuck waiting for opponent.

Same problem, on iPad. I can hear the music that the game started and then I can hear the sound effects to where the timer is counting down waiting for me to choose whether to start the game or to mulligan but it’s just stuck on the waiting for opponent screen

Same problem on iPad.
iPhone works ok.

I have the same problem on my iPad. I can successfully play on my Android phone.

I also have this problem, but only on my (iOS) phone. Brawl works fine when I use my Windows laptop. Also it's not a new issue, it's been this way for months. But once upon a time, it did work fine.

Same problem, waiting for opponent screen