Reproduction steps:
1. Add support for Korean input on Windows (Settings → Time & Language → Language & region → Add a language → search “Korean” → Next → Install)
2. Switch to Korean input (should see “A” in the system tray after)
3. Open the attached “IN-120809.zip” project
4. Open the “KorIssue” scene (Assets > UIAssets > KorIssue)
5. Enter Play mode
6. Click on the top Text Field to focus on it
7. Click on the “A” in the system tray to switch to Korean input mode
8. In the top Text Field, press "F", "G", "H" and "J" keys sequentially
9. The top Text Field should now display "ㄹ호ㅓ"
10. Click on the bottom Text field to switch focus
11. Click back on the top Text Field (this will select all the text)
12. Press the “F” key
13. Observe the top Text Field

Expected result: The "F" input replaces the whole text with "ㄹ"
Actual result: "ㄹ호ㅓ" is being replaced by "ㄹ호ㄹ"

Reproducible with: 6000.0.56f1, 6000.2.1f1, 6000.3.0a5
Couldn’t test with: 2023.1 - 2023.3.0b1 (Korean input did not work), 6000.0.60f1, 6000.2.8f1, 6000.3.0b6, 6000.4.0a2 (From the 6000.0.57f1, 6000.2.1f1, 6000.3.0a5 version, this error is thrown on the 12th step: “ArgumentOutOfRangeException: startIndex cannot be larger than length of string”)

Reproducible on: Windows 11
Not reproducible on: no other environments tested

Note:
- Does not reproduce in the preview mode in UI Builder