Please change this setting and check
Store -> Configuration -> General -> Content Management -> Use Static URLs for Media Content in
WYSIWYG for Catalog to Yes
But, there are some big drawbacks of doing this. The static URL which is used for media content in WYSIWYG might be non-secure URLs. In such cases, if you change the base URL the URLs on the images will not update to reflect that are two. Although, there is another solution available to fix this problem (not a great alternative in my opinion), but better than changing the static URL option.
Another Solution
Change vendor/magento/module-catalog/Ui/DataProvider/Product/Form/Modifier/Eav.php
Add on line 747:
'add_variables' => false,
'add_widgets' => false,
'add_directives' => true,
'use_container' => true,
'container_class' => 'hor-scroll',
Change lib/web/mage/adminhtml/wysiwyg/tiny_mce/setup.js
Add on line 384 below content = editor.getContent();
content = this.decodeContent(content);
Once done the above-mentioned changes follow the below commands
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento cache:flush
Hope this will help. But, keep in mind that you have just made changes in the core files. Once done, move these file to the local folder.