I never really bothered trying to track down the cause of this problem. Given that this gcode error only comes from Autodesk Fusion 360, not any other CAM package, this is a problem on Autodesk’s end.
Grbl follows the G2 and G3 arc LinuxCNC definitions almost exactly. The error-checking protocol for these two g-codes are actually pretty relaxed, meaning that the arc would have be really screwed up to not pass.
I would ping AutoDesk support about this issue. They are clearly screwing something up in the mm to inch conversion.