This is exactly what is happening. Constant Velocity (CV) is a misnomer. It really should be called Velocity Blending. And CV is highly dependent on the motor tuning. A lower accel/decel value will cause more "rounding" in the blended moves. Higher values stay closer to the actual path. Obviously, you want to set your accel/decel values as high as your hardware will allow to get the best results. For servo systems, I usually tick the accel value to the point where the drive faults out and then back it down 10% for a little bit of margin. For steppers, that point is where you start losing steps. If you play around with it, you may get your system to the point where it really won't make much difference.
I also find it useful to use CV during roughing and constant stop on the last finish pass. Tailoring the tool path in the CAM system can accomplish this. If your machine is acceleration challenged, then you might have to modify the post processor to include G09 (one shot exact stop) for the lone Z feed move. I like that better than using a G04 dwell. But to each their own.
e.g.
G09 G01 Z-20. F2031.24 (here is where it happens)
Steve