Well, that's basically pretty self-explanatory. According to the doc and desired behavior, reparenting a topic by using the save script should work, but there is apparently a problem iff there's a mandatory field in the attached form.
This is important not only because it doesn't comply with the doc, but because reparenting should be streamlined.
Regraded to Urgent, and removed inappropriate "patch" status. We don't know what release it will be in until it has been properly analysed.
This impacts any save where a mandatory field is not explicitly set in the form data to the save.
Closed with the release of 4.0.2