Do you maybe have "volume commands with notes are velocities" unchecked for that particular instrument? This does not just affect volume commands in the pattern but also Qxy.

Pretty certain I didn't, but I have more test cases of things that I saw not working involving MIDI so I will test this again as a sanity check as well.

In testing, I've discovered the real problem. If you simply use the default volume (ie, no volume command) it works fine. Setting the volume with vXX in the volume column, however, breaks it and makes it so that every note has the same volume.

Update: Just discovered this also applies to random volume variation. Using default volume applies it as normal, but adding a volume command in the volume column overwrites it the new volume set there.

Yes, MIDI velocity handling is treated completely separately from how it would work with samples, so there are a lot differenes and inconsistencies. At the moment this is not really a bug but "by design"; it would be nice to make this more consistent with samples but it's not that easy.