Hallo Noel,

at first, it does, later it doesn't.

See this:
1) I choose from the 24 'Built-in' Styles "Pop ballad" (if the song is in 4/4 time) or "Waltz" (if the song is in 3/4 time).
2) I choose File/Import Chords from MIDI File an import the MIDI-File.
3) I edit the file, e.g. change chords, change verses and so on.
THIS "RAW" SONG PLAYS ALWAYS PROPERLY, without any missing notes.

4) I choose a style.
Afterwards SOME MGU-FILES DON'T PLAY PROPERLY, there are melody notes missing (about every thenth or fifteenth song).

To your question: No, the special MGU-Files don't play properly prior to rendering it. I only take notice of it, after I rendered the song.

Your question makes me think, that the problem is not due to the rendering process, but to some problem in Band in Box itself ...

tilia