I finally managed to update the firmware but I had to switch the connections to usb 2.0 and usb 3.0, the firmware update could not be done when the peripherals were connected to usb 3.1
This is how I was finally able to update the firmware of Rog Strix Flare II Animate and Rog Chakram X.
After trying a multitude of times without success for these 2 devices
I managed to update the keyboard by changing the usb port, plugged into the back of the motherboard on usb 3.1 I had the idea of plugging it into usb 2.0 and without plugging in the second keyboard cable used for usb hub and finally the firmware update was successful.
I managed to update the mouse by plugging the 2.4 ghz dongle into a usb port on the front of the case (of course having then switched to usb/off mode for the update).
So it seems that the rear usb 3.1 ports on my Rampage VI Extreme Encore motherboard did not allow the keyboard and mouse firmware to be updated.