Letters ä, ë, ï, ö, ü, ÿ are included in the default font, but the alone umlaut/diaeresis (¨) is not.

I often used an umlaut instead of apostrophe, because I was not holding the Shift key. But when I did it in Minecraft, the umlaut is in the Unicode font, but not in the default font whilst all umlaut-accented letters were included in the default font.