But one comment first: by soft pedal, I presume you mean the left, una corda pedal. It should never, ever be played partially depressed. Ever. Either press it all the way, or release it all the way.If you play with it partially depressed, you will get some odd responses, guaranteed. You'll also, over time, play havoc with the hammers.
Just as the sustain pedal can be effectively used as a gradient as opposed on a binary on/off position, so too can the una corda pedal. It will not play any more havoc on the hammers as normal use would dictate.
It's as though something gets torqued out of position and has to re-center itself after the key shift from the soft pedal action. I had a reputed technician look over the piano mechanisms and he said they all were properly aligned.
I did lengthen the rod by adjusting the adjusting screw at the top of the pedal rod where it meets the lever that moves the key mechanism to take out any free play so that my depressing the pedal immediately shifts the keyboard. If anything else comes to mind, please let me know.