I wonder if it has anything to do with your horizontal rule?
Do you have an
<hr> tag to create the horizontal line under the paragraph? If you do, make sure it has empty lines above and below it.
Could you also try creating a new topic maybe in your staff category to test it out with just the video link, an empty line, and then some text. I want to see if all oneboxing of videos is broken or just that topic.
Can you explain how the “system” user created this post? Was it via the api? Maybe something weird happened with the way it got created.
EDIT: sorry didn’t see
Let us know how that goes.