Exiting mid-game on a board saves your progress, but it doesn't restore the exact board state. When you continue, you get a new board, though any lost hearts remain.
This is partly to prevent cheating the timer, for example, by taking a screenshot, exiting, and then calculating the next move outside the game. But since that's already possible with the pause menu, it doesn't really make much sense right now. :)
Alternatively, I could save the full board state; I'll think about a solution to implement eventually.
If it was the World 3 boss, then you should've gotten the end-game screen instead of the post-game summary, since you actually completed the game. I'll check this today to see if it’s a bug.