It seems to me that the Apple Diagnostics would normally catch a bad battery.
Maybe you have a bad camera, flex or sensor that is pulling excess current. Everything seems normal but it kills the battery.
I would propose doing a test.
- Fully charge the battery
- Close all the apps
- Turn on max brightness and see how long the battery lasts
Then open up your phone, disconnect the cameras, sensors (proximity/ambient light) & the Lightning port and do the same test.
If you see the same result, then there is something wrong on the logic board itself. If you see a noticeable difference, then add one element at a time until you see what causes the drain.