Thank you for contacting us.
I reproduced the issue and logged it in our backlog. At this point I am not able to give you any specific time frame when it will be fixed, but you could subscribe to this public item in the feedback portal
and will be notified immediately for status changes.
Your Telerik points were updated in appreciation of the report.
As a workaround I can suggest you to set the RadRichTextBox's TextRenderingMode property:
Hope this helps.