I don’t think the problem is RTL. It might have something to do with the locale though. I just tried sharing a post to Facebook from my forum and found that it didn’t work. The problem for me was that I was using a plugin that tries to read the Accept-Language header from the request. It seems that when Facebook scrapes the data they don’t set the Accept-Language request header. This was causing an error with the request. When I disabled the plugin Facebook was able to scrape the data again.
I then tried setting my forum’s locale to ‘ar’ and made a post in Arabic (I don’t speak Arabic so it was just random letters and a picture 
) Facebook was able to scrape the data without any problem.
You can test posts here https://developers.facebook.com/tools/debug/og/object/ to see what information Facebook is getting.