The past few days, if I’ve left the Blizzard app idle for some time, the Play Hearthstone button goes gray, and hovering over it produces the tooltip “This game requires you to log into Battle .net in order to play”. When this happens, my account is logged into the app, and I can use the chat features, as well as start any other game with auto-login.
This has happened to me several times over the past few days. Sometimes after coming back to my computer after a few hours, most recently after logging off WoW Classic after about an hour of play. A restart of the Battle .net app temporarily fixes the issue and the play button on Hearthstone will is restored to normal functionality.