BIM: setup changing Preferred working units does not work correctly

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Problem description

Changing `Preferred working units` in the BIM setup dialogue does not work correctly when switching between mm, cm and m

Steps to reproduce:

- open new file
- open BIM_setup dialogue
- change Preferred working units to cm
- close the BIM_setup dialogue with `OK`
- check units set for the document - sometimes the change happens, sometimes not
- repeat with m and mm

See video:

https://github.com/user-attachments/assets/b5c7ff1d-0586-4430-a617-5a0278fc9d80

### Full version info

```shell
OS: Windows 10 build 19045
Architecture: x86_64
Version: 1.1.0dev.39786 (Git) Conda
Build type: Release
Branch: main
Hash: 013eb6e6353ab6e5077ab4696653c9b493442638
Python 3.11.11, Qt 5.15.15, Coin 4.0.3, Vtk 9.3.0, OCC 7.8.1
Locale: English/United States (en_US)
Stylesheet/Theme/QtStyle: FreeCAD Light.qss/FreeCAD Light/Fusion
```

### Subproject(s) affected?

BIM

### Anything else?

The behavior has been discovered and confirmed by two other user in this forum: https://forum.freecad.org/viewtopic.php?t=93687

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct