I have the same issue on Windows. It started a few months back and it gets worse and worse. I basically crash out of every other game I play. To me it is so bad I am considering turning my back on Arena because it is just so frustrating. Nothing helped so far. I reinsalled the game. I keep rebooting my router. It just annoys me so much and since it prevents sideboarding it also affects my win rate along with the ability to test decks for upcoming tournaments.

Whenever the client has server connection issues (weirdly my connection is fine for streams running in the background and loading websites while Arena fails to communicate with it's servers) I get dropped to main menu immedeately and ultimately get a match loss unless I restart the client. Seems like this bug is rare enough that it has been under the radar. I do understand that my ISP somehow fails to provide a perfect connection but since it works with other software, I don't quite get why Arena struggles with it so badly by now.