Pixhack V3 Safety switch LED

Good day all

This is my second Pixhack V3 drone and it is the first time that I am noticing this strange

When I power my drone via USB , the safety switch flashes to indicate disarmed state.
When I connect the main battery while the USB is still connected, it stops flashing. When I disconnect the battery it starts flashing again. When i disconnect the USB and just power my drone from battery, it starts up properly and can even arm but the LED on the safety switch does not flash.

I looked at the parameter-list to see if there is some sort of configuration parameter for this but did not see anything.

Anyone perhaps experienced this ?

Many thanks