1) Configure a user with the default "Autodetect Proxy" setting.
2) At signin screen, select a policy-configured device wide network with a proxy
3) Sign in to the user.
4) Find that you are still connected to this network, but you lose the proxy setting (it's replaced with the autodetected proxy).
5) If the user's proxy setting is set to "allow user to configure" then it's fine - the network-defined proxy is maintained.

I'm thinking we should give priority to the network-defined proxy in pretty much all cases other than an explicitly configured user proxy.