I have come across a document on the MAGIX website "Render Settings for a Blue-ray disc" and of particular interest, given recent discussions, is how to render the audio stream. It seems to suggest that we don't need the AC3 pro codec to burn a 5.1 Blu-Ray if I understand correctly? Sorry a link is not available
https://support2.magix.com/customer/en/request
Just type in "Blu Ray disc" and you will see a link to the document where it describes the settings etc.
I will try with these settings and see what happens