ForitAP - AP_MGMT_VLAN_ID variable missing

Hi,

I have a strange case with some fortiAPs right now. 1x FAP 231G and 1x431F both on 7.6.4. I want to set a VLAN ID to have the management tagged but the variable is missing when typing cfg -s. Then i connected to the HTTPS GUI and the field is also missing. I typed in the command cfg -a AP_MGMT_VLAN_ID=200 on the CLI and I got no error. cfg -c for commit to flash. Nothing happened. Reboot and still no change, the AP won't take the VLAN ID. I have 100 more 231G on this site and also 20x431F and they all have the variable and it works fine. How is this possible? Anyone else encountering this problem?