Incorrect rendering of nestable elements in builder

[re-co]

Hey @Spellhammer ,
I have installed Oxygen 4.8._final and elements API demo plugin.
When I drop nestable example it does not render correctly in builder.

By defualt Oxygen wraps custom element in DIV and nested element in builder will always be first child no matter where echo do_oxygen_element() is placed.

On fronted it outputs correctly.

Expected HTML

<div id="_my-nestable-element">
   <p>Some text</p>
   <div>
   {{ echo do_oxygen_elements()}}
   </div>
</div>

In builder preview

<div id="_my-nestable-element">
   {{ echo do_oxygen_elements()}}
   <p>Some text</p>
   <div>
   </div>
</div>