With KDE (and perhaps some other desktop environments) there are two independent, different resolution values that affect how the GUI is scaled.
The first value is reported by "xdpyinfo | egrep -i resolution" and depends on the actual screen size (at least if the X server can determine it). In my case it returns
resolution: 209x209 dots per inch
The second value is reported by "xrdb -query | egrep -i 'xft' and can be overwritten by KDE's "force font dpi option". In my case it results in
The "force font dpi option" is very often necessary to work around problems in some application that do strange GUI scaling for very high DPI values (firefox is a prime example).
Anyway, LibreOffice looks totally right on my screen with these settings but there is a minor drawback. If I set the zoom to "100%" a DIN-A4 page does not has the size of an actual DIN-A4 page but is too small. If I set 175% the DIN-A4 has the correct size. Nothing of this is very surprising, because 209/120 = 1.74.
However, it would be nive, if LibreOffice would proceed as follows:
(1) For GUI styling (menu bars, dialogs, etc.) use the resolution as being reported by xrdb. (This is already the case.)
(2) But use the resolution as reported by xdpyinfo to scale/zoom the actual page content.
Sounds reasonable -> NEW