Embedding video content is possible only using a simple text editor. Using WYSIWYG dialog data is serialized in the tag which is showing blank white space on the product page.
If IFrame is inserted with a simple text editor it works fine on the product page, but when someone makes an edit with WYSIWYG editor, the code gets also transformed in the code which is showing blank space on the product page.
<iframe width="560" height="315" src="https://www.youtube.com/embed/XXXXXXXXXX" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><img height="315" src="http://hajdi.test/js/tiny_mce/themes/advanced/img/trans.gif" width="560" data-mce-json="{'video':{},'params':{'src':'https://www.youtube.com/embed/XXXXXXXXXX','frameborder':'0'},'hspace':null,'vspace':null,'align':null,'bgcolor':null}" /></p>
Looks like a bug same as described here: https://github.com/magento/magento2/issues/18550
I have exactly the same problem with Magento 1.9.4.4.