Did you know, that on the FortiAP FAP-C24JE, the VLAN ID’s 898 and 899 are reserved for system use?
Or that the FortiAP models FAP-S221E, FAP-S223E, FAP-221E, FAP-222E, FAP-223E and FAP-224E can not work with VLAN ID 97 and 98? I’m sure you already guessed it: These ID’s are reserved for system use.
Lets make an example: You have created a Bridge mode SSID on your FortiGate and you want to bridge the traffic into VLAN ID 97. But since this VLAN is already in use from the AP itself, this SSID will not work. You will be able to configure everything without any hint pointing you to the direction, that you maybe will have troubles using VLAN 97 or 98. Even better: the SSID will be broadcasted as expected. But your Wifi clients will not receive any IP addresses or have any kind of network connectivity.
You can find this information documented in the FortiAP configuration guide: https://docs.fortinet.com/document/fortiap/6.4.1/fortiwifi-and-fortiap-configuration-guide/621215/wireless-network-configuration