One factor is that a step has an infinite derivative, so would be highly sensitive to numerical error. You could try replacing the step with a ramp over a short time, which would result in a finite velocity.
Simscape considers position a local state--it doesn't have the concept of a 'position' in the sense of the 'position' of the south end of your spring, and this can make modeling some mechanisms trickier. So, another possibility is to look at the source code for the spring and write your own spring that takes end positions as inputs.
