Yes, that works as designed, to keep MIDI import always working. Is there a problem with this?

The most obvious use case for keeping them unmapped is to have blank samples when importing MIDIs. Maybe it's a bit of a niche use case, but whatever.

But if it's intentional, why is there be an unmap option in the first place, if it'll be remapped anyway?

keep MIDI import always working

Sure, but Wine doesn't come with gm.dls, so MIDI import is already "broken" by default on a stock Wine configuration.

The most obvious use case for keeping them unmapped is to have blank samples when importing MIDIs. Maybe it's a bit of a niche use case, but whatever.

Is that an actual use case you have or are you just trying to figure out a way why someone might *possibly want the feature to work like that? Like, which concrete problem would this solve during MIDI import?

But if it's intentional, why is there be an unmap option in the first place, if it'll be remapped anyway?

Maybe it should be named "reset to default" and immediately revert to the default mapping (which doesn't need to be GM.DLS - the registry key for DirectMusic could be pointing to any other DLS file). Because as said, I don't see any useful, practical reason for a MIDI library item to be missing.

Sure, but Wine doesn't come with gm.dls, so MIDI import is already "broken" by default on a stock Wine configuration.

One system being broken doesn't imply that all other systems need to be broken as well.

Is that an actual use case you have or are you just trying to figure out a way why someone might *possibly want the feature to work like that? Like, which concrete problem would this solve during MIDI import?

Maybe it should be named "reset to default" and immediately revert to the default mapping

Usually when I import MIDIs I want each instrument to have one blank sample, so I could then load a sample into it. I know I can just make a blank soundfont and use that, but that's a hacky solution.

Plus, if you add "reset to default", I don't see why it would be a problem to add proper unmapping too.
In mptrack.ini, you could store a special value for empty instrument keys, and fallback to the DirectMusic soundfont for missing keys.

Like I said, it's a niche use case, and it's not too important, so I understand if you don't want to add it, but please at least add "reset to default", since the current behaviour is weird.