Hi, I have received a final reply from Kobo, which is:
"This is a follow up regarding your concern. Our team informed that this is expected behaviour. You are welcome to leave this as feedback in our feedback channel for further improvements if you think that would better the user experience."
So, the reported behavior is a design as we've initially joked. IMHO any behavior that changes user setting w/o a user approval is identical to a virus/malware behavior and cannot be justified. But, well... it's not like my opinion matters. I'm glad, as tested by other users here, this behavior only happens on the particular menu screen, so its negative impact is reduced considerably. Still, this design approach reduces my trust in Kobo considerably. Not like there is much trust in any corporation to begin with, though. Microsoft often changes user defined security settings (e.g. turns OFF Restore Point) during Windows update w/o user approval or even any user notification, so it is a sad trend in technology these days. There is no assurance that the next firmware update won't change Sage's Bluetooth behavior to auto-connect at all times. Once the company starts on this road, I don't see any end to it. To again use Microsoft as an example:
(EDIT: THE FOLLOWING TEXT MARKED WITH ITALICS MIGHT BE MY MEMORY PLAYING TRICKS ON ME AND COULD BE ACTUALLY FALSE. IF IT REALLY IS, SORRY FOR POSTING IT)
to use Windows 10 and later you need to agree that MS is legally allowed to delete any files on your computer should they find it appropriate. Where is the end of this road?
My thanks to all users who chipped in with their knowledge and testing.

PH