This article describes limitations with software switches.
FortiAP, FortiWiFi, FortiGate.
It is possible to make an internal zone a part of a software switch in FortiWiFi. as described in related links below.
However, this is not possible in FortiGate.
The scenario outlined in this article is ideal when assigning DHCP leases to wireless clients via a DHCP server that is located in a LAN.
In order to make an internal interface part of a software switch, it is important to get rid of any IP address assigned to it and remove any references to this interface.
See the example below, where they are marked in yellow.