Falcosoft wrote on Yesterday, 14:54:

You should consider loops as a continuous way of the playback of the patterns. That is it's the same as if you copied the patterns where the loop starts to the end of the song. So if the pattern where the loop start does not define its own volume level it inherits the volume settings (and of course others as well) from the previous ones. So if no effort is done to prevent differences it's more likely than not that after a loop the patterns will sound differently (volume, pan etc. differences) since the previous patterns before the looped ones differ. You have to make sure that you insert the volume, pan etc. settings to the beginning of the 1st looped pattern to make sure they have the same values as inherited from the previous patterns at 1st playback time without the loop.

I was thinking the same thing, but this bug doesn’t occur when the loop repeats—maybe right at the start, but actually right in the middle of the loop. You can already see it clearly in the waveform: it’s the central portion that’s messed up. The file was generated with OpenMPT. Problems occur with original UMX too.

RetroGC wrote on Yesterday, 22:19:

I was thinking the same thing, but this bug doesn’t occur when the loop repeats—maybe right at the start, but actually right in the middle of the loop. You can already see it clearly in the waveform: it’s the central portion that’s messed up. The file was generated with OpenMPT. Problems occur with original UMX too.

I'm not sure that we are talking about the same thing. And I'm also not sure that I'm seeing what you saw 😀
I can only see that the pan/balance of the left/right channels change. At the 1st part the left/right channels are balanced while at the 2nd part the the left channel is quieter while the right channel is louder.
To tell you the truth I do not think I can tell anything more than I have already told above.

But this behaviour should'nt depend on loop. This is deus ex ost if you want to check files (umx or Impulse tracker).
Aldo