Same here. AA and vw nav can not work together SADJust tried it as you suggested. All settings exactly as you described.
Doesn't work for me.
VW nav works fine and goes into dark mode automatically.
Android Auto only changes to Dark mode when overall VW settings are changed to dark. No auto change for Google Maps via Android Auto.
Had the car from new for 12 months tried repeatedly with many permutations - nothing works.
Will leave in VW dark mode and use Google Maps in dark mode 24/7 as I e had to do over the past year.