It's a hack of the forwarded-header, it's an "internal" and undocumented option.
You can produce the same behavior with trustedIPs and an "impossible" IP.
https://docs.traefik.io/configuration/entrypoints/#forwarded-header
The option will be applied only on one entrypoint, but if it's your "main" entrypoint it's an issue.
I think we need to add something to do that on v2 