Yes, there are indeed different Qt versions

On the working RPI5 there's just Qt5 (default from distribution)
On the arch-based machine there are both Qt5 and Qt6 libraries (qmake reports using Qt 5.15.16)
There were probably some mistake when installing pyside6 that pulled some more libraries than needed. Other programs are still able to call the print dialog though, not sure which of them are doing that using python.
It's anyway a minor issue at the moment since it only happens on this "lab" machine that could have a quite "dirty" environment due to previous experiments.