You have a file that's an access controlled link created through the Nextcloud repository and embedded in Moodle as a resource (mod_resource). If you view this file while not logged in to that Nextcloud server you should get a login popup. However, if the file is an image, the login popup never gets displayed. The request for pluginfile.php does return the HTML that shows the plugin popup. But it's being loaded into an <img> tag. So you as the user don't see anything: neither the image nor a login popup.