Qt6: render resolution too low

Since the update to Qt6 (commit e306cf22), the PDF is rendered at a too low resolution, resulting in a pixelated PDF. The issue only affects the PDF content, all GUI elements are perfectly sharp (The text box, the checkbox and the page number label). Thus, this seems to be a katarakt issue and not a Qt6-issue and also not a configuration issue on my system:

If I downgrade to commit e225e0d0 (one commit before the upgrade to Qt6), the rendering is perfectly fine:

Both Screenshots use the default configuration (no ~/.config/katarakt.ini and no ~/.config/katarakt-stylesheet.qss). The qt-related environment variables on my system are:

QT4_IM_MODULE=ibus
QT_IM_MODULE=ibus
QT_QPA_PLATFORMTHEME=qt5gtk2

The resolution of my screen is 3840x2160, but I don't have any hidpi-settings as far as I know. I will test the qt6-version later today on my "low"-dpi system (1920x1200 resolution).