Message is being shown incorrect after Preview

1. Choose any topic
2. Press Answer button
3. Type a text consisting of 2 lines and including bb-codes
[b]To search and not give up!
To find and get it hidden![/b]
4. Press Preview button

As a result 2 lines have been united in one.

5. Press Answer button.

As a result, the message is being shown incorrect (bb-codes doesn't work correct ):

<span style="font-weight: bold;">To search and not give up!
To find and get it hidden!</span>

See example on a screenshot (the first message was created without using Preview function,the second message - with using Preview)