Hi Mark,
OK. Here goes.
1. Opened the midi file in Composer and began playing it. (I looked at channel assignments first this time; RH was auto and LH was ch 1.)
2. At about measure 40, I decided to split the hands. The treble notes were playing all right, at this point.
3. Did the split hands, selecting piano LH.
4. While it was working to do this, I decided it might be as easy to just use shift up to move the notes to the RH staff, so I undid the shift hands. (I checked track setup this time before ctrl z-ing, and found the first staff still auto and the now-labeled RH and LH staves were assigned channel 1. All notes were playing at this poing.)
5. After the undo, I began in measure 29 to move RH notes to the upper staff, changing clef symbols as needed.
6. After doing several measures, I went back to 29 and play them back, noticing right away the RH staff was not playing.
7. After a perplexed several minutes, I decided to try changing the staff on auto channel to ch 1. Now all notes played.

I'm not convinced there is a bug. If you recreate these steps, notice that my name appears under the staff on auto. Since all the piano notes are on the one staff to start with, I'm guessing that the only reason for that staff being created during the import of the midi file is that text. The strange thing is that the only place my name is enter in the original document is in composition properties, on the composer and copyright lines.

So, rather than a bug, I think I did something I shouldn't have--tried to use the staff for notes when it was there just to contain text.

Hope this helps with something,
mgj