When you set a countdown timer it stops the screen from locking.
Because of this, the stop button and back button can both be accidentally triggered by items of clothing or an accident touch. Something made easy seeing as 40% of the screen can either stop or wipe the timer.
Why is it not set up like the stopwatch? Which requires a double tap to start and stop? It’s so maddening that Garmin persists in these silly little design flaws. Made worse that they nailed the behaviour with the stopwatch widget.