Flipper control behave strangely when its content is a ProgressBar

<materialDesign:Flipper Width="128" Height="128">
    <materialDesign:Flipper.FrontContent>
        <Grid Width="128" Height="128"
              Background="{x:Null}">
            <ProgressBar Width="128" Height="128"
                         HorizontalAlignment="Left" VerticalAlignment="Top"
                         Style="{StaticResource MaterialDesignCircularProgressBar}"
                         IsIndeterminate="True"
                         Value="0" />
        </Grid>
    </materialDesign:Flipper.FrontContent>
    <materialDesign:Flipper.BackContent>
        <materialDesign:PackIcon Width="128" Height="128"
                                 Kind="Sack" />
    </materialDesign:Flipper.BackContent>
</materialDesign:Flipper>

The ProgressBar keeps scaling even if I specified the size of Grid and ProgressBar (as the code above)

Environment

VS 2019
.NET 5.0
MaterialDesignThemes 4.3.0(from nuget)

在 .NET 5 上使用 MaterialDesignThemes 4.3.0 和提供的 XAML 重现该问题。首先，将包含圆形 ProgressBar 的 Flipper 与所述的 128×128 尺寸进行比较。完成标准是 ProgressBar 保持在指定大小以内，而不是继续缩放。