Maybe not the right solution, but I have the opposite problem. I always create drawings in metric in my CAD software (not AutoCAD), want to mill in metric but when I open the DXF file it always says inches in my CAM. The easy solution for me is that before I import a DXF file I open it in a text editor, for example in Notepad, look for the variable group $MEASUREMENT and change the second parameter from 0 to 1. I guess in your case it must be changed from 1 to 0. Try it if this solves your issue at least temporarily. The DXF files are simple text files, change only the $MEASUREMENT and don't touch anything else in the file, don't remove any space, tabs or anything else. It may not be your problem at all, but maybe worth a try. Also, I don't know how drawing scale is connected to drawing units in AutoCAD, perhaps those are different settings.