In the svg header I set the font-family to monospace

<svg xmlns ="http://www.w3.org/2000/svg"
xmlns:xlink ="http://www.w3.org/1999/xlink" version="1.1"
width=" 29.70cm" height=" 21.00cm"
viewBox="0.0 -21.00 29.70 21.00"
font-family="monospace" font-size="0.25"
stroke-width="0.02">

Under Ubuntu 18.0.4 LTS with KDE I can now trigger the bug, if changing something in the KDE Font settings. The important difference can be found in the kdeglobals:

diff   .config_without_bug/kdeglobals    .config_with_bug/kdeglobals
111,113c111,113
< XftAntialias=true
< XftHintStyle=hintmedium
< XftSubPixel=none
—
> fixed=Hack,9,-1,5,50,0,0,0,0,0,Regular
> font=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular
> menuFont=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular
114a115,116
> smallestReadableFont=Noto Sans,8,-1,5,50,0,0,0,0,0,Regular
> toolBarFont=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular
127a130
> activeFont=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular

Under Ubuntu the "monospace" font is redirected to "Ubuntu Mono". But instead of using the "Ubuntu Mono" a default font (in this case "Noto Sans") is used.

I tried multiple different programs from the system e.g. kwrite and self written e.g. with qt 4.8. None of them has the bug.

In Qt 5.14.2 I can also set the font e.g. for a QLabel to "monospace" and it works correctly.

An older binary linked against qt 5.10.0 does not show this bug with QGraphicsSvgItem.

I did not check so far, if this bug only applies to QGraphicsSvgItem or also to QGraphicsTextItem.

Using "Ubuntu Mono" instead of "monospace" in the svg also works correctly.