Comment 5 for bug 1544339

Yes, this would be very much a time-sensitive, and not-always-reproducible bug. It hinges on whether every bit of the proxy is up to date with the archive, so that Release and Release.gpg are in sync or out of sync, which is why InRelease could work around it pretty well.

Another option is to make sure the proxy does not cache any files for the archive, although that will negate the speedups you could see from caching.