Well, the fact remains for me anyway - I cannot transpose from C major:
In a new document with solo piano, I enter some notes.
I set the key to C major via popover.
I select all, go to transpose dialog, and transpose on major second up.
Music moves, but the key does not change - I cannot even tick that checkbox.
If I now add a transposing instrument like alto saxophone and then select all and go to transpose, the checkbox will be tickable, and I can transpose to D major.
Also, if I have set the key to D major, I can transpose back to C and C will now be transposable after this little excursion.
If I am missing the obvious, please enlighten me. It should be possible to transpose from C without having to resort to workarounds.