When building an app with app extensions, as part of the app build process we build the app extensions:

where we get the path to the resulting app extension (appex) from the "Build" target output, because we've overridden "Build" to return that for app extensions:

However, when building from Windows, the "Build" target is overridden again:

so the logic specific to app extensions to return the path to the appex doesn't work.