Hi,
JA Masthead module is loading the image_fulltext or the default image: /images/joomlart/hero/bg-1.jpg in Single article page and its not a problem from our product.
If you want to show the image from JA Masthead module without image_fulltext from article. You will need to customize code, in this case and you can consider my tweak
Open templates/ja_mono/html/com_content/article/default.php file
find and change
$masthead = JLayoutHelper::render('joomla.content.masthead', array('item' => $this->item, 'params' => $params, 'imageBg' => $imagesBg,
'topInfo'=>$topInfo, 'icons' => $icons, 'view' => $this, 'event' => $this->item->event));
$doc->setBuffer($masthead, array('type' => 'modules', 'name' => 'masthead', 'title' => ''));
to
/*$masthead = JLayoutHelper::render('joomla.content.masthead', array('item' => $this->item, 'params' => $params, 'imageBg' => $imagesBg,
'topInfo'=>$topInfo, 'icons' => $icons, 'view' => $this, 'event' => $this->item->event));
$doc->setBuffer($masthead, array('type' => 'modules', 'name' => 'masthead', 'title' => ''));*/
Regards