Hello, We understand how frustrating it can be to discover your AOD has turned off. There are some steps to try to correct this behavior.
1. Try clearing the app cache. https://www.samsung.com/us/support/answer/ANS00077611/
2. Go to Settings>Apps>AlwaysOnDisplay.
Check that the app is Unrestricted.
3. Clear cache partition and repair apps in the recovery menu.