update through squid proxy fails

PortableApps 16.0.1 running on non internet connected machine, connecting to a squid server on a machine with internet access, that connects to a parent squid server over a ssh tunel on another machine with internet access.

Updater gets list of needed updated fine, then fails to download anything.

Running wireshark on the machine with PortableApps and the parent squid proxy (didn't ran it in the squid in the middle machine) I see, for each application needing update, two requests of the form, back to back
GET http://portableapps.com/redirect/?s=p&a=application&d=sfpa&wv=7&f=applic... HTTP/1.1
that get replies like
HTTP/1.1 303 See Other
Location: https://downloads.sourceforge.net/portableapps/application_version.paf.exe
but there is never a request to actually get the file from the redirected location.

Is PortableApps supposed to issue a get to the redirect, or is it expecting to just get it, i.e. should squid resolve it and send the file back?