I had some time to draw up what I think the problem is, by what I’m hearing - haven’t been out to the garage to cut anything, but this looks to me like the issue that is happening.
In this sketch, you can see a base curve (the left one) and two “offset” curves - one is offset radially by 1/8 inch, one is offset linearly by 1/8 inch.
Since I use Fusion, I don’t know the process for determining toolpaths that you are using - with Fusion, I select the feature and the bit diameter and it does the rest to determine what path the bit should take. You can see in this case, the path that the bit needs to take is a radial offset (that is, the diameter the toolpath actually takes is different than that of the feature we are dealing with). If we were to offset only linearly (that is the blue line, which has a 1/8 offset in the x dimension), you can see the bit follows the same arc as the feature (the left black line), but because of the bit’s diameter, the cut ends up having a different shape.
Can someone confirm that my impression of how your toolpaths are being created is correct? I can confirm 100% that Fusion would cut the right hand black arc, resulting in the shape of the cut that you ultimately expect. This would then cause even more issues for more complex and variable shapes.
Hope that leads to more success - and I do want to get into the garage, but we just had a new arrival and things are of course busy!