That system has been a bugbear of mine since I had my first in 2012, but they sorted it on 2018 models.
Up to 2018 models, the dimming was activated by turning the lights on (manual or auto via the dashboard light sensor). You could override it by turning the stalk on the dials, or use the display setting on the satnav itself (auto/day/night). But if you ever needed lights on after (say) a summer thunderstorm leaving lots of spray, the display would also be dark. Post 2018 models have the light sensors independant for headlights and dashboard so you can have the lights on with the display still bright (unless the cabin is dark).
Alan