I'm not sure why this isn't receiving much attention, but the other two threads I saw on this topic were locked, with barely any reply from devs or mods.
Not sure if the culture at Opera is to ignore requests from users to improve the browsing experience, at the cost of logic and common sense.

Since incognito mode operates in a new window, having each tab use the same favicon makes no sense, and there's a reason why practically every other browser shows favicons in incognito windows as well.