iOS 10: Custom alert sound is not working (iOS plays default alert sound for all reminders and timers)

This is a iOS 10 bug that affects apps that have been updated from the App Store.

The only resolution is to restart your iOS device. To do so press and hold the Sleep/Wake button at the top (iPhone 5s or earlier) or right side (iPhone 6 and iPad) of your device until a red slider appears on screen. Slide your finger across the slider to turn your device off.

After the screen becomes completely black, turn your device on again by pressing and holding the Sleep/Wake button until the Apple logo appears.