Undo Beam middle produces broken beaming

1. Open attached score.
2. Click bar (selecting it).
3. In 'Beam Properties', double-click 'Beam middle'.
4. 'Undo'.

Result: The third note in the first triplet is no longer beamed to the other two notes.

Using MuseScore 2.0 Nightly Build (5627) - Mac 10.7.4.