Title setting in EntityViewController::view() is not working right

There is some code in EntityViewController::view() that attempts to set the page title:

    if ($_entity instanceof ContentEntityInterface) {
      $label_field = $_entity->getEntityType()->getKey('label');
      if ($label_field && $_entity->getFieldDefinition($label_field)->getDisplayOptions('view')) {
        $build = $this->entityManager->getTranslationFromContext($_entity)
          ->get($label_field)
          ->view($view_mode);
        $page['#title'] = drupal_render($build, TRUE);

What this is doing is getting the label field's current field display settings for the current view mode, and using it to set the #title in $page, which is the overall render array that this method is returning.

So, there are two cases:

a) You have this field currently set to be displayed with some formatter. In this case, it is rendered and put into $page[#title]. Then it is also rendered again during the field rendering, and (as you have specified in the field settings), it is displayed again. So if the entity is being displayed on its own page, you're going to get the title field displayed once in an H1 as the Drupal page title (this is also the HTML title of the page, which is what you want). But then you will get the title displayed again in the fields section, as some kind of div as a field. This is undesirable -- you don't want it displayed twice.

Not only that, but if for some reason you had a label set up for the title field in your display settings, it would put something like "Title: Foo" as the HTML page title, the H1 Drupal page title, and then again in the field area. Ooops.

b) You have this field currently set to be hidden. In this case, when it is rendered using the field settings, the result is an empty string. So if the entity is being displayed on its own page, you get nothing for the HTML page title, and also nothing for the H1 Drupal page title. And it isn't displayed as its own field in the fields section. This is also undesirable.

So... I think that currently all you are left with is that you need to have a title callback in your route for entity viewing that sets the page title in a different way, not involving the field formatting settings.