This could be a problem with unicode normalisation.
Because there are 2 ways to generate an ä:
- Code point “ä”
- ¨ + [Combined Grapheme Joiner] + a
Try renaming the Kräuter file on your phone. When you delete the “ä”, do you have to press backspace twice? After renaming the file name it “Kräuter” again. Does it work now?