I also had this problem, tried everything including forcing my phone to dark mode and setting the CarPlay display in Dark Mode, but maps would always display in Light Mode as soon as I started navigating (even at night).
I have however found a solution - I set my phone to schedule Dark Mode automatically from sunset to sunrise, and lo and behold my CarPlay maps now come up in Dark Mode at sunset when my phone changes its 'base setting'. This allows me to have my lights on during the day with light maps (my Audi A3 doesn't automatically change the display to Dark Mode as the Pioneer head unit is aftermarket) and when the maps do change to Dark Mode at sunset, the inside of the car is dark enough for night mode to be effective.
Hope this workaround works for others as well.