Created attachment 108336 [details] Example of wrong bold font rendering in clock widget Since upgrading ubuntu to Artful, that comes with libfreetype 2.8.0 all font-hinting was broken. Updating libfreetype 2.8.1 fixed normal font hinting, but still broke bold fonts in window titles. Minutes ago, plasma was upgraded to 5.11 and hinting the bold title font was fixed. Bold fonts in plasma clock widget e.g. and kmail mail list still render unhinted though.
This issue could be in Qt or in the font rendering libraries (freetype or Mesa). Plasma itself has no code to render fonts. If this is still reproducible with a Tumbleweed live ISO or any other rolling release ISO image, I suggest to report this issue directly to Qt developers via https://bugreports.qt.io/