MacBook Pro 2018 in 24/7 nightmode - cursor on external monitor not in nightmode
One quick thing you could try is resetting the NVRAM.

To reset the NVRAM on your model MacBook Pro follow these steps: 


Shut down your machine. Yes, a full shut down, not just logging out.
Press the power button and then immediately press the commandoptionpr keys. 
Keep these keys pressed down for at least 20 seconds!
Let go of the keys and let your MBP reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).