Steps:

1) Create or open a Quick 3D QML file with Model which source is e.g. "#Cube".

2) Open it in designer property editor and modify the source property.

3) Writing "#Sphere" and pressing enter is not working, it causes the field to clear. Changing the value in "Set Binding" dialog doesn't work either. Looking the code in the text editor side shows 'source: ""'

Model source should support all built-in primitives: https://doc-snapshots.qt.io/qt6-dev/qml-qtquick3d-model.html#source-prop