I stumbled upon this for a "Unable to find a corresponding IP address" issue with fail2ban.
I noticed that there's a typo at the end of your failregex ( although yours seem to work fine )
so for completeness here's the latest official one
failregex = pure-ftpd(?:\[\d+\])?: \(.+?@<HOST>\) \[WARNING\] %(__errmsg)s \[.+\]\s*$