Transpose to same key loses signature (0.9.6 regression)

Steps to reproduce bug
1. Open promenade demo
2. Edit > Select All
3. Notes > Transpose
4. Press "OK"

Expected behavior: Nothing should change

Actual behavior: Key signature changes from two flats to no flats.

MuseScore version: r. 2919 nightly. Bug not present in 0.9.6 beta 2 (r. 2811).

(Operating System: Windows 7)