Sounds like something that needs to be addressed in Mach. I've got a tormach that runs Mach3 - It's got the analog spindle. They run it as step/direction though, and I assume that the step signal must just go in to some charge pump or something and get fed in to the VFD as an analog signal.... but same deal. An increase in speed is nice and gentle. A decrease bumps, thumps, etc. I assume with a true servo, this would be a pretty violent thing.
I notice that on the VFD, I have to set the decel pot carefully. I needed a replacement VFD back a few months ago (under warranty YAY!) and I had to tune it. The Accel pot could be set almost anywhere, because Mach was ramping it up, but if I set the Decel pot wrong, when I reduced speed a lot or stopped, the VFD would error out with an overvolt. The motor was generating too much juice in the process of slowing down too quickly, and was literally acting like a generator. They all (motors) do this of course, so it needs to be addressed.