It looks like that naming a field ID "post_title" is not anymore working as it should.
Basically I used to change the field ID to "post_title" when I wanted to use that field as the tile of the post, but lately this doesn't work anymore.
It happens the the field is is not seen as the post tile so I need to type the post title twice, in the title itself and in the field, that is now visible as a custom field. Before it wasn't visibile as it was acting as the post title.
Sorry if it is a little bit confusing, bit I didn't know how to better explain.