• .NET Core Version: Microsoft.NETCore.App 3.1.9
• Windows version: 2004
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No

Problem description:
Hi, when I place the TextBox inside the popup, it will input the letter only when the next letter is entered. Please take a look at the gif https://i.imgur.com/z9fNvHE.gif .

Actual behavior:
The letter was inputted only then I pressed the next key. Reproduced in IME mode with the Korean language.

Expected behavior:
The letter should be inputted immediately

Minimal repro:

   <Grid>
        <Popup IsOpen="True" Width="100" Height="100" Placement="Center">
            <Border Background="Red">
                <TextBox Margin="5"/>
            </Border>
        </Popup>
    </Grid>

NetCore_Sample.zip
Framework4_8_Sample.zip