Forti60F 7.4.5 Speed LED off

We purchased some FortiGate 60F, some were on 6.4.6 and some were on 7.2, except for one FortiGate that was upgraded to 7.4.5. We noticed that the Fortigate on 7.4.5 has its speed LED of LAN 1-4 turned off occasionally (happened twice in 2 months of the Fortigate in use). We tried rebooting the FortiGate and factory reset, but the speed LED did not turn back on.
We shut down the Fortigate and replaced it with a spare device, and the next day when we turned on the Fortigate, the speed LED of LAN ports is on again. With the device's LAN speed ports back up when turned on, the hardware test will pass, and RMA will most likely be rejected. Anyone experienced in the same or other versions, and what are the solutions?
