Previews don't render hotlinked images and images using HTML img tags. This has caused confusion on wikis with $wgAllowImageTag and $wgEnableImageWhitelist set to true. See https://phabricator.miraheze.org/T4316#82239.
Topic on 2017 wikitext editor/Feedback
Thank you for bringing this issue to Wikimedia's Phabricator, Whatamidoing.