Hidden instruments re-appear when clicking outside the stave of visible ones
1. Create 4/4 Piano (bottom) and Voice (top) Score.
2. Enter a note into the first piano bar.
3. Hide Empty Staves.
4. In Note Entry, click on the highest F on the treble stave of the piano, or above.
Result: Voice re-appears with such note.
Using MuseScore 1.0.0 pre-release 2 (3949) and 10.4.11.