QT apps do not scale like other apps

When I connect to a monitor with a 2560x1440 resolution, non-QT apps are not scaled (e.g. Gajim) but QT apps (e.g. Keepassxc, Quassel) are scaled with a factor 2. It is so whether I set the scaling to auto or to 100% in the MATE display settings.

I see in a shell that the variable QT_SCALE_FACTOR is set to 2. If I set it to 1 and start keepasxc from that shell, it is not scaling anymore. Searching on trisquel.info, I found https://trisquel.info/fr/forum/font-size-not-controlled-mate-settings (it was me, but I can't remember which hardware I used then). I installed qt5-gtk-platformtheme, no more scaling problem.

Unless this would break something, or there is another solution, my suggestion would be that qt5-gtk-platformtheme is installed by default.