it looks like the static mode is causing this issue, the ondemand is causing another one.
[15-Dec-2017 15:46:01.353826] DEBUG: pid 889, fpm_pctl_perform_idle_server_maintenance(), line 363: [pool plesk] currently 0 active children, 3 spare children
[15-Dec-2017 15:46:02.355159] DEBUG: pid 889...