Well the point is that ä is one character, not two. It shouldn't be "a with two dots on it", it should be ä. It's its own letter with its own key on Swedish keyboards. MacOS apparently normalizes it to be two characters, and then somewhere in the publishing chain it gets mangled and end up as a¨. I have no doubt that it looked ok on the author's Mac.

It's been a while since I last saw it, but it wasn't because of the font since it was published on a Swedish newspaper's website and other texts worked fine.