If admin uses a custom domain, different from the default website, the WYSIWYGWYSIWYG
image-selector will return malformed media directives, causing broken links to images.
Expected directive: {{media url="wysiwyg/image.jpg"}}{{media url="wysiwyg/image.jpg"}}
Returned directive: {{media url="http://backend.dev/media/wysiwyg/image.jpg"}}{{media url="http://backend.dev/media/wysiwyg/image.jpg"}}
This is known Magento 2 bug https://github.com/magento/magento2/issues/16427, but maybe there are some workarounds until Magento 2.3 will be released?