If you wouldn’t mind filing a bug report, it’d help yourself and the community. The KDE team is responsive and can help you sort through things, but we also need every bug report we can get right now in order to iron out Plasma 5.11.95 issues before the Leap 15 release in a few months. Provided that Tumbleweed is updated and running 5.11.95.
Also, is the brightness slider faded out or just not displayed at all? Because you have to click the box next to it to be able to adjust the slider.