It seems you use apt-cacher-ng, please try if the issue reproduces without it, and if so, if it happens with a known good proxy like squid (e.g. using deb-squid-proxy).
It seems you use apt-cacher-ng, please try if the issue reproduces without it, and if so, if it happens with a known good proxy like squid (e.g. using deb-squid-proxy).