Unable to enter umlaut Characters with compose key

If I try to enter a ü into a string within the qtcreator source code it does not work. It works when I copy&paste it from somewhere else. on the otherhand if I type an ß it works.

ü (needs two keystrokes for one char)
ß (needs one keystroke for one char)

Reproduce:

• attache US Keyboard to a Mac
• open QtCreator, open a file
• press Option+u, u (Option is the Alt Key on a normal keyboard)