Thanks for the great feedback! We're glad you liked and it helps us improve the game! :D
The mechanic you noticed is intentional: new frogs are only born when there are two alive on the screen. If you throw both into the fire, reproduction stops, but the game only ends when the bonfire goes out.
However, you are absolutely right that this specific condition isn't clear enough. We'll be implementing a clearer game over screen and more explicit feedback to make sure players understand why the game ended.
Thanks again!