If you are using WP-Fail2Ban, WP Fail2Ban Redux, their equivalents on Drupal and Joomla, or various other third-party filters then these may stop working after upgrading from an earlier 0.10.x release of Fail2ban.
As noted in the change log, the default flag for the systemd backend was changed to SYSTEM_ONLY(4) in order to ignore user session files that could prevent "Too many open files" errors on a lot of user sessions. As a consequence anything that writes its log output as a non-root user may no longer be visible to Fail2ban by default. Web applications like WordPress usually write their log outputs as a non-root user!
The fix is to change this line in either the relevant [jail_name] section of /etc/fail2ban/jail.local or in /etc/fail2ban/jail.d/jail_name.local
backend = systemd
to
backend = systemd[journalflags=1]