Correct styles not loaded on including ResourceDictionary directly in Application.Resources merged dictionaries

[siagupta0202]

Description

Styles are not applied correctly when trying to add a ResourceDictionary directly in the Application.Resources merged dictionaries as following :

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/PresentationFramework.Fluent;component/Themes/Fluent.Light.xaml" />

            <ResourceDictionary>
                  <!-- Content of Dictionary1.xaml -->
            </ResourceDictionary>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

When I add the resources as above, the first ComboBox and ComboBoxItem (in the sample app) do not pick up any style and defaults to Aero2, however subsequent ComboBox and ComboBoxItem pick the correct style.

Now, when I add the content of resource dictionary in a separate file and refer that file in Application.Resources as follows, the issue does not occur.
Eg:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/PresentationFramework.Fluent;component/Themes/Fluent.Light.xaml" />

            <ResourceDictionary Source="/Dictionary1.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Reproduction Steps

1. Run the application attached and notice that the left ComboBox and the first element of the left ComboBox are aero styled whereas the rest are fluent styled
2. Go to App.xaml, remove the second ResourceDictionary from Application.Resources and instead add <ResourceDictiony Source="/Dictionary1.xaml" />
3. Run the app again and notice that the results produced are now correct.

Expected behavior

Style should be correctly applied

Actual behavior

Style is not getting applied correctly

Regression?

No, this issue is reproducible on .NET 8 as well

Known Workarounds

No response

Impact

No response

Configuration

No response

Other information

No response

[miloush]

Duplicate of #2286?