1. Open the desktop effects kcm
2. Scroll the list of effects one mouse wheel unit down
--> Font corruption happens
If you scroll another wheel unit, everything looks fine again. Another wheel unit, and the font is garbled again. And so forth. The same goes for the application launcher. Choose a category with many entries and start scrolling per mouse wheel. Here the corruption is even more severe usually, as all smaller labels become garbled gray blocks... even those outside of the list but still in the application launcher window like "Computer" or "Leave".
I tested this on a fresh Gentoo x86-64 system w/ KDE Frameworks 5.25.0, Plasma 5.7.4, Freetype 2.6.5 and Qt 5.6.1 as well as today's 5.6 git. Xorg uses nvidia driver 370.23. KWIN Compositor is using OpenGL 3.1. But neither of those latter ones make any difference.
I also downloaded a fresh OpenSuSE Tumbleweed KDE Live iso and ran it in a virtual machine. The problem was 100% reproducible there as well.
A different user also reported this on the KDE Bugzilla for a Fedora system: https://bugs.kde.org/show_bug.cgi?id=358472
If there is anything at all I can do to help track the cause of this issue down, please let me know since thus far I have failed to really locate the root issue on my own.
I have attached two sample screenshots from my system. The one from the application launcher is a case when the corruption was not as severe. I will get another later today that shows the severe case.