After the update have you tried restarting your phone? As a best practice, I always wipe the device cache partition after a OS update. It gets rid of any possibly corrupted temporary values. This does not remove any app nor user settings. Have you wiped the device cache partition?
[update edit] I just updated my Fold 4 to UI 5.1. I can confirm that all the notification icons are one color. This seems to be a UI 5.1 feature/bug. You can avoid the feature/bug by:
- Open the phone settings
- Click on Wallpaper and style
- Click on Color palette and turn it off.