QComboBox does initially not display items (Debian 12, Qt 6.5.2)

On Linux Debian 12, with 

Besturingssysteem: Debian GNU/Linux 12
KDE Plasma-versie: 5.27.5
Versie van KDE-Frameworks: 5.103.0
Qt-versie: 5.15.8
Kernel-versie: 6.1.0-11-amd64 (64-bits)
Grafisch platform: Wayland
Processors: 4 × 11th Gen Intel® Core™ i3-1115G4 @ 3.00GHz
Geheugen: 30,9 GiB RAM
Grafische processor: Mesa Intel® UHD Graphics
Fabrikant: Intel(R) Client Systems
Productnaam: NUC11PAHi3
Systeemversie: M68306-501

Qt 6.5.2, the QComboBox dropdown does not work.

As can be seen in the image below.

This is the example project 'Delegate Widget Mapper´.