Changes to /etc/pure-ftpd.conf get overwritten at some point due to a natural configuration change. There are storage configuration files to hold changes so they are added to the configuration file build when the configuration file is updated.
The main configuration file is locate at /var/cpanel/conf/pureftpd/main and needs to be copied to /var/cpanel/conf/pureftpd/local and updated with the deisred changes.
cp -ax /var/cpanel/conf/pureftpd/main /var/cpanel/conf/pureftpd/local
Then modify /var/cpanel/conf/pureftpd/local as needed and then implement those changes with