If you set MaxRetentionSec= for systemd journald, some messages might be lost in the log files that rsyslog saves

Issue

Some messages are missing in /var/log/messages when MaxRetentionSec= in systemd journald is set.

# cat /etc/systemd/journald.conf
:
MaxRetentionSec=1day
MaxFileSec=1day

Example:

Given that we set up /etc/systemd/journald.conf as follows:

# cat /etc/systemd/journald.conf
:
MaxRetentionSec=1day
MaxFileSec=1day

And that we run the following script:

#!/bin/bash
I=1
while [ $I -lt 1000 ]
do
  logger " -$I"
  I=`expr $I + 1`
done

we expect that the messages are fully saved into /var/log/messages:

# cat /var/log/messages
:
Oct  2 18:34:28 xxxxxx root[23340]: -601
Oct  2 18:34:28 xxxxxx root[23342]: -602
Oct  2 18:34:28 xxxxxx root[23344]: -603
Oct  2 18:34:28 xxxxxx root[23346]: -604
Oct  2 18:34:28 xxxxxx rsyslogd[3805]: imjournal: journal files changed, reloading...  [v8.1911.0-3.el8 try https://www.rsyslog.com/e/0 ]
Oct  2 18:34:28 xxxxxx root[23539]: -605
Oct  2 18:34:28 xxxxxx root[23541]: -606
Oct  2 18:34:28 xxxxxx root[23543]: -607
Oct  2 18:34:28 xxxxxx root[23545]: -608

However, we sometimes observe that some messages are missing in /var/log/messages:

# cat /var/log/messages
:
Oct  2 18:34:28 xxxxxx root[23338]: -505
Oct  2 18:34:28 xxxxxx root[23340]: -506
Oct  2 18:34:28 xxxxxx root[23342]: -507
Oct  2 18:34:28 xxxxxx root[23344]: -508
Oct  2 18:34:28 xxxxxx root[23346]: -509
Oct  2 18:34:28 xxxxxx rsyslogd[3805]: imjournal: journal files changed, reloading...  [v8.1911.0-3.el8 try https://www.rsyslog.com/e/0 ]
Oct  2 18:34:28 xxxxxx root[23539]: -605
Oct  2 18:34:28 xxxxxx root[23541]: -606
Oct  2 18:34:28 xxxxxx root[23543]: -607
Oct  2 18:34:28 xxxxxx root[23545]: -608

where the messages from -510 to -604 are missing.