I finally understand a bug on OSX which bugged me for a long time. When I stored a file with the name äöü on and HFS+ volume and wanted to access this file via ä[tab] is would work. It was eventually fixed.

But know I know why this happened. HFS+ normalise the letter and ä becomes two characters, the a and the umlaut dots. And bash didn't know that or didn't care, so it so no file with an ä but with an a at the beginning.

Is this analysis correct? I so hope that APFS will be a little bit more sane.