Ref T10809. Currently, both the proxy and target may mutate URIs (rewriting "svn+ssh://x/diffusion/Y/" to a path on disk).
I believe this previously worked by fate/chance/luck since both URI variants contain the repository information, but the algorithms were tightened up recently with callsign removal.
Stop rewriting them if we're the intracluster proxy -- they only need to be rewritten on the target host.