Many users are facing issue with Apple maps that the CarPlay Maps not going into night mode. CarPlay works great but navigating with Maps at night is difficult because the screen is so bright as night mode does not turns on automatically to reduce the light intensity. This very irritating and annoying bug in carPlay as Night mode doesn't work on the car screen. The issue is reported with iOS 10/iOS11 as well.
Few possible solution to fix the carplay stuck in daymode are as follow:
Fix 1 : Turn dashboard light below 100%
The fix is making sure the dashboard light dimmer is NOT set at 100%. Take it down one notch and night mode on MAPS will kick right in. Turned the dimmer for the car down to about 75%, the dimming for the head unit kicked in, as did the night mode for CarPlay. Not sure if this is a default for CarPlay, but you might play around with the car dimmer.
Fix 2: tap on the navigation
A "fix," to push it into night mode is to punch in directions to anywhere and when navigation starts it goes into night mode, then cancel navigation. This should suddenly kick night mode in CarPlay to fix the issue.