You are using Stemgen and you want to contribute? That's great! There are many ways to contribute to Stemgen, and we appreciate all of them.
Lossless stems are not working on Windows. This is forcing users to encode in AAC. It looks like there is an issue with https://github.com/NativeInstruments/ni-media when reading ALAC stems.
You can help other users by answering questions on GitHub issues and in the Native Instruments forum. You can also help by improving the documentation.