This is the Discover/End Turn bug. It has been a serious problem for months. Blizzard created the problem with a patch yet has not acknowledged the problem’s existence. It has also been mentioned on Reddit several times, but keep in mind, many persons there downvote posts about bugs that the devs should see, so it may lack visibility there.
As noted in the first link, this affects all multiplayer game modes, including Battlegrounds, and all platforms — Mac, PC, and Mobile — when it occurs and is absolutely a showstopper.
Dealing With the Discover Bug
In my experience, if the bug does not happen in a game, it won’t happen at all so long as the application remains open. Thus, the “workaround” is to start a game of Brawl (to give your opponent a free pack if you concede) or casual and try to force the bug, preferrably with a discover card on turn 1. (Obviously, if Brawl costs gold to enter due to a terrible anti-customer casino marketing campaign, don’t use Brawl, use casual.) If you get the bug, restart the application. If you don’t, you can concede and play the “real” game. This can take several restarts.