I have been silent on this, as I use a Nomad, not a Shapoko, but you might want to see: No G2/G3 ARC Movements?
The C3D post for Inventor does not produce G2/G3 (unless it has been updated since last august… I just tried it), and I see the same effect on my Nomad.
If I adjust the tolerance the effect is reduced, but, obviously, the file size increases.
I don’t know if CC does it the same way, of if it used G2 and G3, but the $12 parameter in GRBL sets the interpolation precision. Take a look at the g-code file you are running to see if there are G2/G3 commands. If so, this can handle it. If not, tighten the tolerance on the operation AND when you post (there is a configuration setting in the post popup window for this in Inventor and, I think, Fusion)
Or, replace the operations in the g-code file by hand with G2 (or G3) as appropriate. This will take a little work, but might be worth it to you.