Changing actual time signature causes score corruption

1. Load attached score
2. Select second bar in Vocal part
3. Measure Properties
4. Change actual time signature to 21/8
5. OK

Expected behaviour: Actual Time Signature should change to 21/8
Actual behaviour: Actual Time Signature remains 20/8 but adds an additional quaver rest to the end of the bar, thus causing a corruption - further editing produced this mess:

Marked critical due to the corruption

MuseScore 2.0 r5592/WindowsXP Pro SP3