Application scaling randomly broken

Ever since a recent update of my KDE stable Manjaro, application scaling is randomly broken. xsettingsd is installed, using an X11 session. My display is 4k, scale in Display Configuration is set to 200%, Legacy Applications (X11) is set to “scaled by the system”.

Firefox and other GTK+ applications seemingly randomly have the correct interface scaling, or too large or too small. The Firefox UI is sometimes double as big as it should be, making the browser almost unusable, though never too small. Lutris and the gnome-disks application are randomly too small.

This would suggest a problem with GTK+ scaling only, but I also see random scaling problems with QT applications. The Manjaro Add/Remove Software app for example is sometimes too small.

I tried switching to Wayland, and I’m also seeing Firefox randomly with too large a UI.

Thanks for any help!