This is not a local configuration issue. Please see the steps to reproduce: besides the 'sudo apt install squid-deb-proxy squid-deb-proxy-client' there is no additional configuration present.
The issue is not in "how your proxy configuration will interact with your sources.list" but in "apt using proxy in order to access local resources on the machine where apt is installed", it is not an access to whatever in sources.list that triggers the errors, but an attempt to access localhost through the proxy.
This is not a local configuration issue. Please see the steps to reproduce: besides the 'sudo apt install squid-deb-proxy squid-deb- proxy-client' there is no additional configuration present.
The issue is not in "how your proxy configuration will interact with your sources.list" but in "apt using proxy in order to access local resources on the machine where apt is installed", it is not an access to whatever in sources.list that triggers the errors, but an attempt to access localhost through the proxy.