Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
I do not but in the past this was never an issue and the image always posted to fb. It even still posts to twitter .
EB also posts cover images to fb and tw.
What has changed?
If i recall correctly, that is Facebook changed this behaviour in the past (End of last year 2017) for prevent other 3rd party app share the fake news in Facebook.
Facebook now always look for the article page opengraph tag, if those article page doesn't have render any opengraph tag, Facebook will randomly pick one of the image on the site.
Perhaps you can try find one of the system plugin which can render opengraph tag in Joomla article page, it might solve this issue.