I had this same problem, LL fans would not light up in Far Cry 5.
Found a solution for now, until Corsair can find a real patch.
Not sure if a clean install is needed but I did it just to be sure.
(Optional?) Do a clean uninstall/install as outlined by Corsair Calico Jack
Open iCUE and click on the profiles tab. In the context menu next to the profiles tab, click the arrows icon to import a profile. Click the "..." button and navigate to the GameSdkEffects folder in the iCUE install directory (Should be "C:\Program Files (x86)\Corsair\CORSAIR iCUE Software\GameSdkEffects") Open a folder and select a profile.
Seems like any profile works. Tried with various FC5 and even a Division 2 profile works. Click "Import" to import the profile. Open Far Cry 5 and watch your fans light up again.
TL;DR: Importing a predefined iCUE profile used by a game seems to fix the fans not lighting up.
Unfortunately, if you remove the profile from your iCUE profiles it breaks the fans again. Just gotta live with having one of your profiles being one of the predefined ones. Not a big deal IMHO, most are pretty cool by themselves and I just moved that one to the bottom of the list.
Btw, "deleting" a profile in iCUE does NOT delete the actual .cueprofile file so feel free to delete if you want.