I couldnt reproduce it on a plain new song , so you have to figure it out with my track.
After you loaded the song, go on track 7 and instrument 8.
Play the song and keep your ear to track 7.
Go to the instrument envelope of Instrument 8.
If you move the endpoint of the Volume-Loop to the left, you will hear all three notes, if you put it back to the end you will hear only those instrument, which has 50+ ( i think ) velocity, others are muted.
If you put a point near the end-range marker,and pull it up to anything beside 0,00% you suddenly hear all three notes again.
Could also be, that i messed up anything, thats why i couldnt reproduce it.