Reducing the acceleration is likely the culprit. In CV mode, the lower the acceleration, the more rounding you'll get. If acceleration low, Mach3 needs to start turning the corner sooner in order to try and maintain constant velocity.
If you look at the toolpaths in Aspire, you'll see that the toolpath "rolls around" the corner, which is common practice.
The problem is that there is no 90° corner in the toolpath, so setting Mach3 to Exact Stop at 89° has no effect.
The "corner" is a 90° arc tangent to the sides, so there is no corner.
You'll need to increase acceleration, or cut slower.
I don't think it's possible to get toolpaths from Aspire that don't "roll around" the corner, but I'm not positive.