Hi Rhyxali,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with Night Shift on your Mac. We're happy to provide some information to assist.
If your referring to the feature working for external displays, check out the recommendations on the third-party website here: Fixing Night Shift incompatibility with external monitors
You can also double check all settings and requirements with the article and guide pages below.
Change Displays Night Shift preferences on Mac
Guide page: Use Night Shift on Mac and the article: Use Night Shift on your Mac
Connect an external display to your iMac
If the issue continues, test after booting to safe mode and normally. How to use safe mode on your Mac
We'd also recommend checking a new user account to see if the issue is user related. Change Users & Groups User preferences on Mac
If you continue to have any trouble with the feature, reach out to Apple support directly to further assist.
Contact Apple Support
Best!