That's about the way it is with the current software. Unlike nearly every other system, instead of using the input as the only, and absolute trigger for chuff, RP is using the input to get a speed sample. Most of the time RP is still using a software method to time the chuff.
This might be OK in the smaller scales, but in Large Scales it is very noticeable, and annoying!
I came from a much less capable control system combined with Phoenix sound cards. While the RP sounds are comparable, the chuff timing is not. I do lots of slow speed operating / switching, and when running Phoenix, I could judge loco speed by sound without looking at the loco. I lost that luxury when converting to RailPro.
I am hopeful that based on feedback, Ring will update the use of the trigger to be an absolute and we will no longer have locos chuffing while standing still!