Technical Tip: FortiAP not auto adjusting the Tx power and always operating on the Maximum power set in the range

Check the ForitAP profile's auto-power-target setting which should not be empty. 

config wireless-controller wtp-profile

    edit "FAP231F-Test"
        config platform
            set type 231F
            set ddscan enable
        end
        set handoff-sta-thresh 55
        set ap-country DK
        set allowaccess https ssh snmp
        set frequency-handoff enable
            config radio-1
                set band 802.11ax
                set auto-power-level enable
                set auto-power-high 25
                set auto-power-low 6
                set auto-power-target ''
                set darrp enable
                set vap-all manual
                set vaps "FRDWLAN" "RD-GUEST"
                set vap1 "FRDWLAN"
                set vap2 "RD-GUEST"
                set channel "1" "6" "11"
            end
            config radio-2
                set band 802.11ax,ac-only
                set channel-bonding 40MHz
                set auto-power-level enable
                set auto-power-low 11
                set auto-power-target ''  <----------- It should not be empty. The default is -70.
                set darrp enable
                set vap-all manual
                set vaps "FRDWLAN" "RD-GUEST"
                set vap1 "FRDWLAN"
                set vap2 "RD-GUEST"
                set channel "36" "44" "52" "60" "100" "108" "116" "132"
            end
            config radio-3
                set mode monitor
                set wids-profile "default-wids-apscan-enabled"

FortiAP would only adjust power to a lower value if other FortiAP can hear it high than -70 (This is a default setting on the AP profile where -70 is the value set on 'set auto-power-target -70').

Note:

auto-power-target is used to configure the target Received Signal Strength Indicator (RSSI) level that the FortiAP should try to maintain for its clients through automatic transmit power adjustment.