In my case, haproxy will close the idle connection , 5 minutes after the last message . haproxy send FIN to fluentd actively , and fluentd become close_wait , in this time, fluentd send a message, haproxy will send RST immediately，after this tcp connection is close. The message you just sent is missing.
My log collection system like this:
fluent => haproxy ==> rsyslog
In my case, haproxy will close the idle connection , 5 minutes after the last message . haproxy send FIN to fluentd actively , and fluentd become close_wait , in this time, fluentd send a message, haproxy will send RST immediately，after this tcp connection is close. The message you just sent is missing.