Strange Behavior Opening .dxf in Carbide Create

I’m brand new to CNC stuff and I have created a .dxf file in Inventor Professional 2018. When I try to open the .dxf file in Carbide Create, something strange is happening. What’s happening is a chamfered rectangle in my design is being converted to a circle. Here is the .dxf with what it is supposed to look like.
StepperCase.dxf (206.3 KB)
However, this is what it looks like in Carbide Create.
Any help would be appreciated. Thanks.

Weird. It appears okay when opened in Inkscape, but even after joining all paths and combining into a single composite path, it opens as your image shows.

Drew in the geometry to clip off the corners and then opened it. Attached.

StepperCase.c2d (10.5 KB)

Thank you for the quick response. I agree, it is weird. I uploaded the .c2d file you gave me however the dimensions are off. The tiny holes are supposed to be 3mm but they appear 4mm in your file.

Please scale the file by the overall dimensions and then you should be good.