bbCode deletion doesn't work

1. Choose any topic, press "Answer" button
2. Type any text, using bbCode (for example, [i]I want to delete bbCode![/i])
3. Press "Preview" button (the text with correctly demonstrated bbCode should be shown - I want to delete bbCode!)
4. Press "Preview" button once more to have the possibility to change the text
5. Delete bbCodes (the plain text I want to delete bbCode! should remain)
6. Press "Answer" button

As a result, bbCode hasn't been deleted, the text is still being demonstrated as if bbCode had been used. (I want to delete bbCode!)
Expected result - plain text without bbCode is shown (I want to delete bbCode!)

The same mistake happens in Private message.

The mistake doesn't happen without using Preview.