Native VLAN keeps changing from 254 to 1 on FortiSwitch

Hello,

I am kind of new to FortiSwitch. 

We have 3 FortiSwitches in one of our office locations and I lose access to the switches and when I console to the switches, the native VLAN changes from 254 to 1 (I know this sounds weird, but this is what happens)

Here's a high-level overview of our topology:

FortiGate 81F -> Cisco Switch -> FortiSwitch 448e