If you are testing on an Android device with version less than 10 (or higher) you will need to request WRITE_EXTERNAL_STORAGE instead ?
S o l u t i o n 
After two weeks of research:
Look at the path!
In my case:
/storage/emulated/0/DCIM/Camera/IMG_……
was Okay
DCIM/Camera/IMG_
was too short (too less). It was okay for placing the pic in the right folder, but not enough to refreh the gallery-App.
Another important thing is the version. I managed it this way:
(I hope it works 
couldn‘t test properly soon)
good luck!
Emanuel