I noticed some interesting quirks when exporting from Fusion360 to dxf.
First - the sketch needs to be on the XY plane. I have my F360 set up with Z-axis pointing up like conventional machining. The dxf exporter doesn’t like that - it exports from the perspective of the XY plane (so it was exporting just a single line because it was looking at my sketch from the side).
Second - I had problems with scaling. Regardless of whether my document units were inches or millimeters, the exported file was exactly 2.54x the correct scale when using the File/Export protocol.
However, when I right-clicked on the sketch and exported just the sketch to dxf, that was correctly scaled.