Changing Key signature mode has no effect

To reproduce:
1) Create a score and put a key signature.
2) Click that key signature, go to the Inspector panel, under "Key Signature", change "Key signature mode" to anything except "Unknown".
3) Click somewhere empty on the score, or just save the project, and observe that property again.

Results:
The property gets back to "Unknown". It's not just a UI bug, because it also does nothing to the exported MIDI file.

Expected:
The property can be normally set and we get the correct key signature in the exported MIDI file.