I had a similar situation. Even when a successful login takes place, a 'Failed' message appears in the sshd log file (/var/log/messages) It is caused by this regular expression (see Chris S answer):
^%(__prefix_line)sFailed [-/\w]+ for .* from <HOST>(?: port \d*)?(?: ssh\d*)?$
matching lines like this from the /var/log/messages file:
Apr 18 14:07:02 hostname sshd: Failed none for root from 188.8.131.52 port
When a user logs in successfully a 'Failed none' message still appears in the messages file.
To resolve this I changed the sshd_config file (/etc/ssh/sshd_config) to specify
This stops the 'Failed none' message for successful logins, but retains it for login failures.
I also considered updating the sshd.conf fail2ban configuration file and adding something to the 'ignoreregex' line, but updating sshd is a cleaner solution.