I couldn’t find the issues thread that investigates this problem. It turned out there was a bug in the Fusion360 post for Grbl. It was generating arcs that were completely invalid, where the target end point would be no where near the arc radius described. I found some posts worked fine, others had this problem. I didn’t dig deeper to figure which ones but I recall the current (at the time) default post for Grbl worked. While a user version that has been floating around doesn’t. Hope that helps.
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