I'm not 100% sure this is a bug because I don't currently have a working system with Impulse Tracker to test on, but I'm testing in Modplug Tracker and I think its playback is correct..
Here's the file:
http://www.magicfish.net/chrissy-t2.it (also uploaded to un4seen ftp as chrissy-t2b.it)
It seems that the note-off is triggering a volume jump up in BASS, but it doesn't in Modplug. I think that note-off is supposed to reset the channel volume, but not until the next note is triggered.
This example has an instrument number at the same time as the note off, which I realize is the same thing as a note on (although still cancelled by the note off), but the behavior seems to be the same even if it is removed.
Can someone with Impulse Tracker verify whether the volume jump up is present there?