On my most used Fedora machine, a Dell XPS 9520 laptop, the keyboard backlight setting is not maintained across boots or even sleeps. I have to keep enabling the keyboard backlight setting from time to time - always after a reboot, and probably after coming out of sleep (haven’t kept a close eye on it every time) - using the fn+f5 key (on this laptop).
It was working correctly on Fedora 39 and before that 38 until I upgraded to 40 several weeks ago. I had been waiting for the issue to be fixed since it’s not critical, but the behavior isn’t changing.