If you create a package with a single shortcut that has a target exe that has an icon, you end up with a package using that icon for the Asset icon.
If you create a package with a single shortcut that has a target exe that has no icons (even if the shortcut itself assigns an icon to the shortcut from another source), you end up with a package with no Asset icon. Thus when you go to install the asset icon looks like a blank box.
The PSFLauncher executables do not have icons, so if you swap it in for the shortcut your package ends up with the blank Asset icon.
The MPT does not support editing of the Asset icons, at least in an easy way. It seems like important functionality missing for customers contemplating using the Microsoft Store for Business/Education to distribute these apps.