I'm not sure where this report should go, so I'll try to post it here.
In default configuration filter.d/pure-ftpd.conf contains regexp:
failregex = pure-ftpd(?:\[\d+\])?: (.+?@<HOST>) \[WARNING\] %(__errmsg)s \[.+\]$
It doesn't work. It must be:
failregex = pure-ftpd(?:\[\d+\])?: \(.+?@<HOST>\) \[WARNING\] %(__errmsg)s \[.+\]$
(because hostname is printed in parenthesis by pure-ftpd).