As a note, Marcin is asking about the parts being re-rendered, that is, the parts in the render region. I believe you are saying something different than what he was asking which is, you render an image and let it finish, you set a render region, you start rendering within that region, and the parts outside of the region become noisy.
My question still stands - do the parts outside the region remain noisy once the render using regions is complete and the denoising has finished?
I'll also ask whether "Clear VFB between renders" is enabled in the System tab of the Render Settings (if it is, then the VFB will indeed "delete" everything outside the regions when you start the new region render). I know you said it is disabled, but are you sure? :)
Everything is working as expected here - I have Clear VFB disabled; I render to completion (final render); I draw a region; I start another final render; the previously rendered parts stay as final, and the region gets noisy and finally clears up.
And, are you using just one render region, or multiple? Do you have some feathering enabled for the region (your image looks strange, as there is the region being re-rendered, a clean part around that, then a noisy part around that even further away). Is anything changed, such as resolution? IR isn't being used at all at any step?
I keep adding questions, sorry :) You are only using Corona VFB regions, you haven't enabled Max Render Regions at all?