It took several attempts but I finally managed to make "create joint" work. At least I think I did! The only effect I can see is mortises appearing on the "make drawing" tool. Unfortunatly, I am working on a hexagon building and my knee braces enter the posts at a 60 degree angle to the post face. Those mortises do not appear on the drawing. Did I exceed the capability of the tool?

If you see the mortises, then it's working!

Regarding the angled post face: The cutting plane of the joint must align with a face on the mortised timber. What I do in your situation is reduce a section of the timber to create such a face if one does not exist. Here's an example from a 5-sided post used in a prow:



Another option would be to create a special version of the joint where the cutting plane is set to the unusual angle. CB.