Has anyone else had this issue?

When I right-click on a dxf file, AutoCAD is in the Recommended Programs section, but the 'Always use...' tickbox is greyed out.

In Control Panel, Default programs, if I scroll to dxf, SOLIDWORKS 2016 is listed as the current default. When I click 'Change Program' I get the same box as before, with AutoCAD as recommended but Always use greyed out.