For the past few Hearthstone versions, my Android (specifically the Samsung Galaxy S21 5G) has not been automatically rotating in the game when flipping the phone.
Obvious question first : is it only Hearthstone ? Do other apps rotate correctly ?
on my LG I sometimes forget that rotate screen is something I have to turn on in the drag down menu from the top of the screen, and even then, sometimes I have to jostle the phone itself as if the tiny sensor in the phone were like a ball within a ball, and that jostle is to bounce that ball within the ball around despite the rotation should have done so for me. But it makes the phone catch up to the intended orientation I have rotated to.
Ive no idea how a phones orientation sensor actually functions but thats my mental image of how I see it operating. outside of hearthstone the sensitivity of the rotation seems normal and responsive, but within HS it seems like its sensitivity has been cranked down to where only those jostles seem to be able to register the changes.
Looks like a somewhat recent Android update changed the name of the icon in the drop-down settings menu to read Portrait when auto-rotate is off. The update appears to have forced the setting to off by default. Fun!
Thanks for the suggestions to look for the obvious. I don’t use any other apps for rotation, so didn’t think it could be a phone issue. 
Glad to know its now fixed and that in any way I helped lol. GGs!