Hi noahsilv,
Thank you for reaching Microsoft Community.
May I know what connection are you using in connecting the monitor to the Surface Book 2? Is it through a dock or direct connection using USB-C to HDMI cable or etc?
Please try first to clear the display cache of the device. Follow the steps given on this page (Troubleshoot connecting Surface to a second monitor) under Solution 6: Clear your display cache. You may also check the page for more details.
If in case the issue persist, please run these steps on your Surface Book 2:
1: RunSurface Diagnostic Toolkit and check Windows Update
2: Update drivers and firmware
Check what is the current OS Build of your Windows version by going to Settings > System > About.
Go to Download driver and firmware for Surface, choose Surface Book 2 then click Download on the next page.
Choose the .msi file that matches your current OS Build and click Next.
After downloading, run and install the .msi then Restart.
Hope this helps.