Hi, I am not sure if you are using the right tool for the job, The patch tool you are using is with the healing tools and is used as such and tries to "blend" the selection seamlessly hence the value slide.
if what you want to do is move/duplicate something you can use the lasso tool to create a selection to copy to a new layer or use the stamp tool painting on a new layer. Is that what you are trying to do or what exactly?
You are right, It works as it supposed to, I just thought patch tool should only replace selected area with new position, didn't know that it tries to change colors to make seamless blending. Thank you for responce