TCP connection still ESTABLISHED when backend service is down

Traefik Traefik v2 (latest)
1

There is TCP ingressRoute on k8s.
Traefik as a daemon set.
TLS passthrough.
Websocket inside of TLS.

linux device - tcp/tls - traefik2 - tcp/tls backend
Got session ESTABLISHED on linux when backend is down for long (days) time until reboot.
Websocket trying to reconnect and stuck forever.

Under investingation now.