I don't know if this can be improved in a future release, but as of right now, suppose you have a part and it's a regular solid and you mate it in an assembly.

If afterwards you use convert to sheet metal to make it a sheet metal part (for drawing purposes) and you go back into the assembly, all of the mate definitions are lost.

Thankfully the ghost recognition shows you what was where and its not to difficult to re-mate them. But it's hardly smooth, elegant nor fast.