FIXED-IN: KDE Plasma 5.12.2
System Settings' sidebar headers were implemented as orginary Kirigami list items that had enabled: false set, which made them turn an almost unreadable light color. This a hack; it should have used real headers. And it was the wrong hack, as evidenced by the visual breakage.
This patch changes the hack to a different one that works much better. Ideally we would use real Kirigami list headers here, but the current system settings implementation seems to make this a challenge and I'm not sure it's worth it. Willing to be proven wrong, though.