I thought, "pah!, 'tis just your post-processor", and spent the last twenty minutes trying to pursuade Fusion with the UCCNC post to generate a cone - cannot persuade it to move away from G1's - thought there was a hope with the minimum-arc-length but nada.

I still think it's a post-processor issue, and I wouldn't use an stl export (the stl generates a point-mesh resulting in a tessellated object, so doesn't lend itself to arc processing)... but real life (work) interrupts what was otherwise an interesting problem :(