Hi @Damon!
Thanks for the answer. So val gets mapped to the enclosing UID and thus can only be used for field manipulation for the same node. I have tried playing around using ideas from nested nodes but unfortunately this does not work since the Chapter uid for the chapter I want to create for Course does not yet exist and thus there is nothing val(c) can get mapped to.
In my opinion, this is a huge restriction and keeps upsert very limited (for my use cases actually completely useless). I thought that the entire idea of upsert is to handle everything in one request and not splitting up tasks.
However, guess I have to live with this right now. I will change this from Bug to confusing docu and change the bug report on Github to feature request.