Rsyslog have unexpected log entries "rsyslogd: Child xxx has terminated, reaped by main-loop" in /var/log/messages.

Issue

After upgrading rsyslog from rsyslog-7.4.7 to rsyslog-8.24.0, there are some unexpected log entries in /var/log/messages which are listed as following. And the message's level shows as syslog.err.

Apr  5 10:22:26 <syslog.err> rsyslogd: Child 31730 has terminated, reaped by main-loop. [v8.24.0 try http://www.rsyslog.com/e/0 ]
Apr  5 13:09:16 <syslog.err> rsyslogd: Child 20159 has terminated, reaped by main-loop. [v8.24.0 try http://www.rsyslog.com/e/0 ]