Problem accessing git.kernel.org with git v2.33 plus gitproxy

Hi folks,

I've stepped on a problem after upgrading git to v2.33.0. git fetch-pack fails with an error:

        fetch-pack: unexpected disconnect while reading sideband packet

It only happens when I access git.kernel.org over git:// (github over git:// works fine) and if there's a gitproxy configured.

For test I used a dummy gitproxy:

        #!/bin/sh -efu
        socat - "TCP:$1:$2"

It is enough to trigger the issue.

I'm not sure if it's kernel.org problem or git problem.

Has anybody else stepped on the issue? Any clues?