https://bugs.winehq.org/show_bug.cgi?id=42740
Alexandre Julliard julliard@winehq.org changed:
What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |NOTOURBUG
--- Comment #1 from Alexandre Julliard julliard@winehq.org --- This is because Wine used to handle the alpha channel incorrectly, and OpenMPT has a workaround for it. Now that Wine is fixed (by 89d31e42fc64bf78d9390c55baa4fc8ec54539e2) the workaround does the wrong thing.
You can disable the workaround by running OpenMPT with the /noWine flag. I'd suggest filing a bug with OpenMPT asking them to remove the workaround, or to only enable it for Wine < 2.4.