Software: Garmin Connect app for Android, using Google Pixel 3 XL
The back button doesn't always work. It brings you back out of the entire app instead of returning you to the main screen. This is especially apparent if you enter the app by clicking on a notification instead of "manually" starting the app.
Example: I clicked on a notification that I've earned a new badge. It shows me the badge. I then click the back arrow in the top left with the desire to go back to my landing page that has all of my daily stats, etc. It instead minimizes the app. No other buttons or alternative methods are present -- or at least not intuitively so. I go back into the app by clicking on the Garmin Connect icon. It brings me back to the stupid badge page that I can't escape! The only real option at this point is to "swipe up" and kill the app. Then come back into it through the Garmin Connect icon to start it fresh.