On Windows 7 with KB2670838, Direct3D 11.1 is partially available, but (e.g., ID3D11DeviceContext::Map with NO_OVERWRITE flags may behave differently). Qt6 may still initialize D3D11, but certain rendering operations lead to crashes.
If your Windows 7 machine has a GPU with OpenGL 3.3+ drivers (e.g., AMD, NVIDIA), you can launch Qt6 apps with: qt6 windows 7
The most important fact to understand is that On Windows 7 with KB2670838, Direct3D 11