I managed to softlock on a boss level - remaining mines was -1 :)
EDIT: Oh I see, it's not a softlock. OK!
However, in the process of writing that comment, I navigated away from the game... the main menu offered me a 'Continue' option (so I thought, great, it saved my progress!) and hitting it seemed to put me back at the boss level again (though with a different board layout, but that's fine). However it did not reset my hearts, and when I finished the level, it then sent me back to the main menu instead of allowing me to continue my quest...