transposing instruments don't show key signature

When I add a transposing instrument I don't see the key signature. However, when I copy and paste between staves of different tonality, they notes are correct. just the key signature is missing.

I'm using the following version: 32fe6c4

To reproduce this, load the attached score: Invention_#13.mscz
This is a harpsichord score.
Bring up the Add Instrument dialog.
Select soprano saxophone. A new staff is added. The key signature should be D, but it is C.
Now select the 1st measure of the right hand of the harpsichord, and copy it (CMD-C on the mac).
Point to the first measure of the soprano sax score and paste (CMD-V).
Notice that the notes are correct, the first note E has been pasted as F#.
But the key signature is missing.