The brightness keys result in the little brightness panel appearing on screen (see image) but the brightness doesn’t alter. I am running Plasma on a ThinkPad X395. I dual boot with Kubuntu and the brightness works okay. I’ve tried changing the brightness in Settings > Energy Saving > On AC but that doesn’t change anything either.
Any suggestions for possible solutions would be lovely! 
Don’t know the answer myself, but I tweaked the title of your question to describe your issue more closely. Hope this will help others understand your issue better so they can help you out.
I’d suggest trying xbacklight and see what it does.
I had this issue as well… if I recall correctly, it required changing/setting the kernel command line option for acpi_backlight in the grub.cfg file.
Try video, vendor, or native to see if one of those work first. There also may be vendor-specific drivers that you need to install… I didn’t for my case.
