My thought: hey, that is a feature!
I'd be very interested to know if they run case-sensitive by default are not in the final release.
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.