The SEO title you can change is the <title> tag as used in the html head, not the "title" (<h2> normally) as is displayed in the body of the post/page.
AFAIK, RSS feeds send a subject, not a title, and the subject should be the normal post title, i.e. the <h2> not the <title> or as you call it "original post/page title".
So no bug, the plugin is working correctly.
If you show the same title in your feed and in the webpage, I think you can help to consolidate the user experience
Actually, all the more reason for the plugin not to change anything as the title on the page a user will see, is currently the same in RSS as on the website.
Most users won't ever see the <title> from the html head (except may be in search results on Google) as - if it is shown by the browser at all - it is only displayed at the very top in the browser title bar (above the address bar and menus).
Hope this helps.