When writing up a better description for you on the new and enhanced implementation of the Drag & Drop mechanism in 6.4.0 d and later, it was a bit confusing and not providing the results we intended. We finally tracked it down to a bug which caused the Source in the DragDropEventArgs not being consistent. This bug has now been fixed in the upcoming 6.4.0 Release f.
I added a new section in the breaking changes article on the new properties and how they work. They will give you huge optimizations in some scenarios. Please view the new section here.
Please note that until we will release this fix, your results are bound to be confusing, so I recommend that you hang on until the fix will be available and then again take it for a spin and report any problems you may find.