FPM: group is set to www-data
Im not sure if this is on purpose but i think the group ownership in fpm configuration should be change from www-data to the respective client group?
Example on current config: /etc/php/7.1/fpm/pool.d/web1.conf
...
user = web1
group = www-data
...
For your information this PHP version is installed as an "Additional PHP version".