I discovered this bug when listening to the module "Generator" by Hoffman (https://modarchive.org/index.php?request=view_by_moduleid&query=177714) in FooBar with the OpenMPT plugin. When testing, I found that the same bug is present when using OpenMPT 1.29.01, as well as the online player on ModArchive.

Hoffman likes to do a lot of trickery with his patterns and samples to squeeze as much as possible into the limited Protracker .mod-format. One of these is concatenating multiple samples into one, and then using sample offset (command 9xx) to swap between the parts, working around the 31 samples limit.

Another trick is a workaround to the fact that you can only trigger one effect command per line, which is done by triggering one or more effects without triggering a note, then triggering a note, then the effects can be repeated by entering notes without entering the sample #.

In the mentioned module, Hoffman uses these two tricks to combine 4 chord samples into one single sample (#0F), and to combine the effects 9xx (sample offset) with Cxx (volume, which is an effect rather than a separate slot in Protracker patterns).

Observe this pattern sequence:

F#30F093A <--- Trigger the sample F at offset $3A at full volume

• • • 0F093A <--- slot the sample offset $3A for the next triggering of a note
• • • 000000
• • • 000C00 <--- set volume to 0
• • • 000000
      F#3000C40 <--- trigger note F#3 without specifying sample #, at full volume.

Note that the last line specifies a note (F#3), an effect (C40, i.e. volume 64), but the sample is set to zero, i.e. no sample is specified. In Protracker, this triggers samples F at offset $3A and full volume. In OpenMPT however, this line triggers the sample from offset 0, ruining the song.

I do not know for sure, and I am nowhere near Hoffmans proficiency in pattern hacking, but my guess is this bug also applies to other combinations of commands triggered without specifying a sample #.