There is an overlay of the previous pictures on the current one. When trying to render a region.
Sorry for English))
I have noticed this too. When you don’t have the “clear frame buffer between renders” checked, it sometimes ghosts the image. I have also noticed it darken the previous render outside of the region. Not sure if this has anything to do with LUTs / post processing being active or not?
Duplicate or similar already in the bugtracker Renderer for architectural visualization - Corona | Chaos
Good Luck
I can not log in (((Please duplicate who can)
I just tagged your report as “already reported” (which is a “duplicate”) by linking to the bugtracker. There are two similar/same issues like yours logged - so no need for further action 
Good Luck
Thank you
confirmed
(and already on mantis, well described with links to all duplicated 
)