[WinUI3] Binding to Foreground gets broken when element is out of the visual tree.

### Describe the bug

A binding between the Foreground of a custom control and the background of a border gets broken when the custom control is removed and added again to the visual tree.

### Why is this important?

Controls don't change the foreground correctly when the theme is changed.

### Steps to reproduce the bug

[App14.zip](https://github.com/user-attachments/files/22755925/App14.zip)

- Unzip project App14
- Open App14.sln in Visual Studio 2022
- Run project
- Change "Theme" combo-box and notice the bound brush changes accordingly, following theme foreground
- Now press "Add/Remove Custom Control from visual tree"
- Change theme.
- Press button again (notice the brush is not the correct)

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

### NuGet package version

WinUI 3 - Windows App SDK 1.8.1: 1.8.250916003

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_