The TextBox process IME input with delay in Korean

* .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:**
```xaml







```
[NetCore_Sample.zip](https://github.com/dotnet/wpf/files/6187572/NetCoreSample.zip)
[Framework4_8_Sample.zip](https://github.com/dotnet/wpf/files/6187577/Framework4_8_Sample.zip)