I have read a few threads with slightly around this topic by other people, where mostly the answers were around JPG compression v RAW data etc. But my Q is specific to the workflow where the above situation puts me to deal with the patchy noise during editing which is very counterproductive.
Hope someone can see what is the root cause of this.
you are comparing the RAW in the PS ACR dialog with the JPEG preview which the Lightroom import dialog displays. once you co plete the import and view the image in Lightrooms develop module, it should look pretty much the same as in PS, as they share the same RAW engine.
To achieve the same level of noise reduction as the camera applies when creating the JPEG thumbnail, use the noise sliders in the develop settings of Lightroom or ACR. Leaving everything on default is just not good enough, they are set for minimal deviation from the "true" data.
Note: you can actually change the defaults. E.g. i have set different noise reduction levels for the various ISO levels of my camera, along with some other tweaks i know i like to apply every time, and saved them as defaults, so the get applied immediately on import.