Please follow these steps to reproduce the issue:

1. Open RadEditor Demo: https://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx
2. Choose a word that is present in the text (e.g. "Barcelona").
3. Open "Find And Replace" dialog.
4. Click on the "Replace" tab.
5. Write "Barcelona" or any other chosen word in the "Find" field.
6. Write something in the "Replace With" field (e.g. "Replacement").
7. Select the "Up" Direction in "Search Options".
8. Click "Replace All".

Actual Result: The "The search string was not found." message is displayed and the substrings ("Barcelona") are not replaced.
Expected Result: All occurrences of the substring are replaced.

Note that this functionality seems to be working if the "Down" direction is chosen.
Update: I would like to add that the issue is not only with Replace All. The same bug is happening with "Replace" with both "Up" and Down" directions.