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.
Create the local template file
Then modify /var/cpanel/conf/pureftpd/local with the desired customizations in YAML format:
Pure-FTPD then needs to have the configuration rebuilt:
/scripts/setupftpserver --force pure-ftpd