Deleting empty lines above a heading change the headings format to "normal".

[rangar]

If an empty line that is positioned right above a heading gets deleted, the format of the heading changes to "normal".

[Jakub Ś]

This is native browser behaviour. If you try the same with below code on plain HTML page, you will get same result.

<div style="margin:20px; border:1px solid black; width:500px;" contenteditable="true">	
	<p>&nbsp;</p>
	<h1>test</h1>
</div>

Just put cursor above the header and press Delete. Format will be changed (i.e. H1 changed into P). I'm sorry but there is nothing we can do about it.