You can change the notification light color for each app under "Notification light" using "Use custom value* and adding the app with the + button top right.
I also had the problem that notifications where not working, a clean reinstall of the ROM solved the problem.
I've searched for it so far in vain...
In LOS 16 I had already made these settings and in LOS 17.1 these changes were even applied. I have now deleted the personal changes of the LED color and blinking speed and entered them again.
The LED now works again with all apps except FairEmail. The developer of FE has added a workaround which causes his app to respond to the LED again, but he suspects that our LOS 17.1 does not work properly at this point.