Missing element if the same key is given to two form items (conflict with metatag)

I've found that if the metatag module is enabled and you try to manage a "node edit form" then the node title field is missing. I've defined the priority to major since it makes impossible to use this module on node forms.

Here is the information displayed at rel-register/nojs/article_node_form . There is no other "fields" with a "title" machine name
Element: "Title", Machine name: "title", Group: "Meta tags"

So the origin of this issues seems to comes from the _rel_recursive_elements way of flattening the form elements. Maybe something like this would be a good start:

if ($parent) $key = $parent . "_" . $key;

But I've no ideas of what this can implies (and even if it's remotely correct).