Many thanks tuxd3v ! I'll retest with the values you suggested today and let you know how it goes 
[edit] with your suggested values set in /etc/ats.conf
I am seeing same behaviour, the fan engages for a split second then spools back down. The log below is from ~4 min of ats -t running.

[edit] with your suggested values set in /etc/ats.conf
I am seeing same behaviour, the fan engages for a split second then spools back down. The log below is from ~4 min of ats -t running.
Code:
root@OpenMediaVault:~# ats -t
info:'SYSTEM' Table
info: 'BOARD' Table
info: 'NAME' = ROCKPRO64
info: 'CPU' = RK3399
info: 'THERMAL0_CTL' = /sys/class/thermal/thermal_zone0/temp
info: 'THERMAL1_CTL' = /sys/class/thermal/thermal_zone1/temp
info: 'PWM_CTL' = /sys/class/hwmon/hwmon0/pwm1
info: 'MAX_CONTINUOUS_THERMAL_TEMP' = 69
info: 'MIN_CONTINUOUS_THERMAL_TEMP' = 60
info: 'MAX_PWM' = 255
info: 'MIN_PWM' = 125
info: 'ALWAYS_ON' = false
info: 'PROFILE_NAME' = profile0
info: 'PROFILE' = 2
info:'Pratio' timers
info: 'Pratio[ -20 - 60 [' = 0
info: 'Pratio[ 60 ]' = 125
info: 'Pratio[ 61 ]' = 139
info: 'Pratio[ 62 ]' = 153
info: 'Pratio[ 63 ]' = 168
info: 'Pratio[ 64 ]' = 182
info: 'Pratio[ 65 ]' = 197
info: 'Pratio[ 66 ]' = 211
info: 'Pratio[ 67 ]' = 226
info: 'Pratio[ 68 ]' = 240
info: 'Pratio[ 69 ]' = 255
info: 'Pratio[ 69 - 70 [' = 255
Stopping for[ seconds ]............... 3
CPU Temperature[ max 70 °C ].......... 0
GPU Temperature[ max 70 °C ].......... 0
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------
Running for[ seconds ]................ 0
CPU Temperature[ max 70 °C ].......... 45
GPU Temperature[ max 70 °C ].......... 46
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------
Stopping for[ seconds ]............... 120
CPU Temperature[ max 70 °C ].......... 45
GPU Temperature[ max 70 °C ].......... 46
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------
Running for[ seconds ]................ 0
CPU Temperature[ max 70 °C ].......... 44
GPU Temperature[ max 70 °C ].......... 46
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------
Stopping for[ seconds ]............... 120
CPU Temperature[ max 70 °C ].......... 44
GPU Temperature[ max 70 °C ].......... 46
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------
Running for[ seconds ]................ 0
CPU Temperature[ max 70 °C ].......... 43
GPU Temperature[ max 70 °C ].......... 45
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------
Stopping for[ seconds ]............... 120
CPU Temperature[ max 70 °C ].......... 43
GPU Temperature[ max 70 °C ].......... 45
Fan PWM Duty Cycle value[ 0 - 255 ]... 0
--------------------