Gotcha. But I have the same problem with the new layout. I added a new track with 4 measures of midi data right at the beginning. In the project window the midi part itself is only 4 bars long so there is no midi data after that. In my initial example the same is true for Voc1, there is no midi data beyond bar 16. Now the new part shows notes on the first 4 bars and then the staff continues empty for the rest of the piece.
I also created a new test project with 2 midi tracks and the results were even stranger. Track 1 has a midi part filled with midi notes in bars 1-32. Track 2 has a midi part filled with midi notes in bars 1-8. I selected both parts and opened in the Score Editor in page mode. As I expected I saw 2 staves for 32 measures, the second one only showing notes on bars 1-8. Then I went to Auto Layout, selected Hide Empty Staves and hit OK. Then the score changed so that bars 1-8 showed for both tracks as expected, then only the staff for track 1 showed for bars 9-24, also expected. But for bars 25-32 the staff for the empty track 2 reappeared.