[Bug] Unable to modify the PlacementProperty of flyout in DropDownButton

Describe the bug

I tried to modify the Placement property on the <ContextMenu Placement="Top"> in xaml, but it didn't take effect.

Here is the complete code:

<ui:DropDownButton Content="Hello" Icon="{ui:SymbolIcon Fluent24}">
    <ui:DropDownButton.Flyout>
        <ContextMenu Placement="Top">
            <MenuItem Header="Add" />
            <MenuItem Header="Remove" />
        </ContextMenu>
    </ui:DropDownButton.Flyout>
</ui:DropDownButton>

After checking, I found that there is such code in DropDownButtons.cs

  private void OnPreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
  {
      if (_contextMenu is null)
      {
          return;
      }

      _contextMenu.SetCurrentValue(MinWidthProperty, ActualWidth);
      _contextMenu.SetCurrentValue(ContextMenu.PlacementTargetProperty, this);
      _contextMenu.SetCurrentValue(
          ContextMenu.PlacementProperty,
          System.Windows.Controls.Primitives.PlacementMode.Bottom
      );
      _contextMenu.SetCurrentValue(ContextMenu.IsOpenProperty, true);
  }

Maybe

    _contextMenu.SetCurrentValue(
        ContextMenu.PlacementProperty,
        System.Windows.Controls.Primitives.PlacementMode.Bottom
    );

should be removed or modified.

To Reproduce

Define a DropDownButton in the xaml file and modify the placement of Flyout.

Sample code:

<ui:DropDownButton Content="Hello">
    <ui:DropDownButton.Flyout>
        <ContextMenu Placement="Top">
            <MenuItem Header="Add" />
            <MenuItem Header="Remove" />
        </ContextMenu>
    </ui:DropDownButton.Flyout>
</ui:DropDownButton>

Expected behavior

The placement property of flyout can be modified through xaml.

Screenshots

No response

OS version

Windows 11 Pro 25H2, build 26200.7922

.NET version

.net framework 4.8.1

WPF-UI NuGet version

WPFUI 4.2.0

Additional context

No response

Open DropDownButtons.cs and inspect OnPreviewMouseLeftButtonUp, where the flyout placement is set before it opens. Reproduce the provided DropDownButton and ContextMenu Placement="Top" example, then verify that the XAML placement is honored while the button still opens with its expected sizing and target.