rhel7: Why doesn't systemd journald honor `SystemKeepFree` option?

Issue

• I have configured systemd-journald to avoid growing the journal files if the amount of free disk space in the filesystem is less than a specific amount:

  # grep -i systemkeepfree /etc/systemd/journald.conf 
  SystemKeepFree=1850M
  

• Despite the amount of free disk space being higher than the threshold:

  # df 
  Filesystem            1K-blocks   Used Available Use% Mounted on
  /dev/mapper/rhel-var    2037760 142760   1895000   8% /var
  #
  # journalctl --disk-usage
  Archived and active journals take up 50.0M on disk.
  

• The journal files eventually grow beyond the free space threshold:

  # df
  Filesystem            1K-blocks   Used Available Use% Mounted on
  /dev/mapper/rhel-var    2037760 333008   1704752  17% /var
  #
  # journalctl --disk-usage
  Archived and active journals take up 200.0M on disk.