The light/dark mode transition animation missing in macOS 15.5

The missing transition animation when switching between light and dark mode in macOS Sequoia 15.2 is likely due to the "Reduce Motion" setting being enabled. 

To restore the animation, you need to disable this setting in System Settings. 

Here's how to fix it: 

1. Open System Settings: Click the Apple menu () in the top-left corner of your screen and select "System Settings".
2. Navigate to Accessibility: In the sidebar, click on "Accessibility".
3. Go to Display: On the right, click on "Display".
4. Disable Reduce Motion: Locate the "Reduce Motion" toggle and turn it off.
5. Test: Try switching between light and dark mode again. The animation should now be visible.

Reference with the steps : Stop or reduce onscreen motion on Mac - Apple Support

You can also contact Apple support Contact Apple Support - Apple Support

Since the issue still persist even after reinstalling macOS I would suggest to reach out with the Apple Support this might need further investigation

https://getsupport.apple.com/

If that's the case since Sequoia 15.2, Apple probably removed the feature and is unlikely to come back. I'm always in dark mode so I haven't noticed.

Things change. I can't speak for Apple.

Possibly.