This isn't a bug, but a peculiarity how Oktalyzer's 8-channel mode works (paired channels require 7-bit samples but the sample playing on that channel is an 8-bit sample). You will also not hear that voice in Oktalyzer itself. I was in contact with an Oktalyzer user when I implemented this in OpenMPT (at his request, actually), and we both theorized why the demo song was released like that; Based on the name (Grundlagen = fundamentals), we think that it might be because it was supposed to teach the basics of how the program works, to show the user that 8-bit samples don't work on paired channels.

Checking the video, I just realized you are referring to the other demo song, which doesn't have the paired channel issue. Instead, it uses Oktalyzer's extended arpeggio effects which are currently not supported by OpenMPT.

I could finally manage to get the original Oktalyzer v1.57 up and running and i can confirm the corresponding voice has 'A57' effects.
So, as you already know the bug feel free to handle the ticket (keep/close).

Thanks!

I'll keep it open as a reminder but it's unlikely that there will be an immediate fix (new effect commands need to be implemented).

As of r19869, the missing arpeggio commands are worked around by transposing the notes in a way so that they still sound acceptable with standard MOD arpeggio. We may still add Oktalyzer-style arpeggios later, but this at least makes the track almost sound as intended. Test builds including this fix will be available from https://builds.openmpt.org/builds/ within a few hours.

I'll keep this issue open as a reminder to implement proper effect commands for this.