Could be the maps not the signs although only 40 fixed signs with 7am-7pm and the electronic ones that change?I am a bit surprised that your car reads the electronic 60 sign, as mine appears to ignore all electronic signs and just apply map data in their place, though it recognizes all non-electronic speed signs now.
If not the electronic ones I assume it sit on 40km to it sees a new 60km sign not revert to map?
Although who knows how it programmed. The only signs not 40km are electronic.