Hi Pat,
That's a great bug report - the video really made it easy to see what problem you are having.
First of all I should explain the terminology that Crescendo uses, in contrast to what you are using: It refers to 'measures' as 'bars', 'bars' as 'bar lines', 'lines' as 'staves', and 'charts' as 'scores' or 'pieces'. I'm not saying that Crescendo is correct here, just clearing up what I'm talking about.
Now I think what's going on is that when you change the key, the key signature takes up more space, and Crescendo decides that there isn't enough room for all 4 bars on the second staff, so the last of these 5 bars gets pushed out, and forms its own 1-bar staff. This may be something we need to work on - maybe there is a better way for Crescendo to behave in this case. But I think it should still be reasonably easy to solve your problem: after you change the key, right-click on your new 1-bar staff, and choose Bar > To Previous Staff. This will move it back to where it was originally. If this option is disabled in the menu, it probably means there isn't room for this bar on the previous staff, so make some room and try again.
Alex