RST-ACK sent by the server after there is a FIN-ACK from the client

in the vserver's server side tcp profile, ensure that keepalive is enabled.

https://my.f5.com/manage/s/article/K13004262

at least in will prevent in between firewall and server's tcp stack to mark the tcp connection as stale.

server's application layer might not be affected by tcp keepalive.
it may have idle timeout based on application transaction inactivity.