WPF flow document TextRange doesn't replace existing content right

### Description

## Current Situation

When writing two or more [``](https://learn.microsoft.com/en-us/dotnet/api/system.windows.documents.section) elements to a [`TextRange`](https://learn.microsoft.com/en-us/dotnet/api/system.windows.documents.textrange) object, supposed to replace the **whole flow document**, existing [``](https://learn.microsoft.com/en-us/dotnet/api/system.windows.documents.section) objects in a flow document control are not getting replaced. Instead, the new sections are *appended* to the existing sections:

            

## Desired Situation

When replacing Flow Document content in a Flow Document control using …
```xaml
new TextRange(TextBox.Document.ContentStart, TextBox.Document.ContentEnd).Load(ms, DataFormats.Xaml);
```

… *all* previous content should be replaced.

## Reproduction Steps

Please refer to this MRE for details:
https://github.com/SetTrend/WpfFlowDocumentFlaw

### Impact

**Can't use.**

### Configuration

- .NET 9.0
- Windows 10x64