during testing I've noticed that the replace result msg box can be a bit misleading.
I've also tested it with your online editor:
You can reproduce it as follows:
1. Enter the demo (link above)
2. Click Search/Replace
3. Change Text to find to: "RadRichTextBox for Silverlight/WPF"
4. Change Replace with to: "mytest"
5. Click Replace
--> text for replacing gets highlighted
6. Click Replace again
--> text will be changed
==> BUT message dialog says:
I think the text of the key 'Documents_FindReplaceDialog_FinishedSearching' (= "Finished searching the document.") should be displayed?!