See patchdemo example at https://defe6ff661.catalyst.wmcloud.org/w/index.php?title=New_York_metropolitan_area
'useParsoid' is set locally, but it is not passed on to the target proxy wiki. This leads to a confused situation, where the HTML isn't parsoid-generated, the "Rendered with Parsoid" footer doesn't appear, etc but "Switch to legacy parser" and "Report Visual Bug" appear in the sidebar as if this were parsoid-generated content.
Probably could be handled by just adding ?useParsoid=0/1 (or ?parsoid=0/1 for the parse API) to the URL used to fetch the target wiki contents.