$form['#title'] doesn't work when the route has a _title_callback

The documentation on page titles says that $form['#title'] is a 'last-chance' to change the title.

However, when the current route has a _title_callback set, this doesn't work.

This is seen for instance when using Drupal\Core\Entity\Routing\DefaultHtmlRouteProvider with a custom entity type, and trying to set the title in the form.