When rendering a content several times in a single runtime, a given embed tag will not be updated and the same content is returned each time.

Looking at "eZ\Publish\Core\FieldType\XmlText\Converter\EmbedToHtml5.php", the first time I render my content, the "render" function returns something like:
<xml>
<section><embed><![CDATA[
MY FIRST EMBED RESULT
]]></embed></section>

And on the second time:
<xml>
<section><embed><![CDATA[
MY FIRST EMBED RESULT
]]>
<![CDATA[
MY SECOND EMBED RESULT
]]></embed></section>

So I made a dirty fix there waiting for your enlightenment.
Thanks in advance !

Romain.