Fail2Ban shows an IP among Found in
/var/log/fail2ban.log but does not actually ban this IP.
Fail2Ban will ban an IP only if it exceeds a certain number of failed login attempts. Every failed login attempt is marked as "Found" in Fail2Ban log. If an IP did not exceed the maximum allowed number of failed login attempts within a certain time period (10 minutes by default), an IP will not be banned. Read more about how Fail2Ban works in Plesk Administrator Guide.
If it is required to modify the number of maximum allowed failed login attempts (max retry) in Fail2Ban settings, change it in Tools&Settings > Fail2Ban > Jails