Hi !! Sorry for the delay. I was not notified by your answer.
The problem still persists. I tried to install a flatpak but the same occurs.
> Does the presence of libreoffice-kde5 make a difference (which is
> in 6.1.x bascially a disguiseed gtk3 with a kde5 file picker)? I see
> below you have it installed (-kde being for kde4 in stretch gets
> replaced with that one.)
> Does it also happen with -kde5 from 6.2.0 (which has a "native" KDE5 plugin
> instead of that gtk3 thingy.)