FCKeditor malfunctions when multiple fields are present and one of them has a teaser

Steps to reproduce:

1. Create custom content type (e.g. TestContent) with multiple text fields.

2. Now try to create first content (TestContent) and into each textarea add the following text (in the source mode)

<p>tekstNheader</p>
<!--break-->
<p>tekstNbody</p>

where N is the number of textarea (1,2,3...).

3. Click preview

4. In the body, instead of

<p>tekst1header</p>
<!--break-->
<p>tekst1body</p>

we get:

<p> tekst3header</p>
<!--break-->
<p> tekst3header</p>
<!--break-->
<p> tekst3header</p>
<!--break-->
<p> tekst3header</p>
<!--break-->
<p>bodybody</p>