I was able to reproduce this unwanted behavior and have logged a new bug on our feedback portal on your behalf - RichTextBox: Korean language input is wrong on Windows 11. You can subscribe to the item to receive status updates. And in appreciation for bringing this to our attention, I have updated your Telerik points.
It looks like this issue is related to the updated version of the Microsoft Input Method Editor (IME). As a workaround, I can propose enabling the Compatibility setting "Previous version of Microsoft IME" (check the attached screenshots for the steps needed) until the issue is resolved.
Should you have any other questions do not hesitate to ask.
Regards,
Vladislav
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