Thank you for the reports. We are aware of this issue, where changing focus from the Hearthstone app on mobile is sending users back to the main hub.
We are working on a fix, but it requires client side changes so it may take some time to submit a new build through iTunes/Google/etc.
Thank you for your patience while we work towards a resolution. Meanwhile, if the game sends you back to the hub while in a game, the quickest fix that I’ve found is just to restart the app immediately. It is definitely not ideal, but can be quicker than trying to queue for a game again.