Yes, I'm using the latest version. I just installed the Wikipedia Beta app and experienced the same issue, but I just discovered the following, applicable to both app iterations: 1. If I open an article page in portrait mode, the left and right margins are the same size. 2. If I open an article in portrait, then rotate the tablet to landscape mode, the left and right margins remain equal. 3. If I open the article in landscape mode first, the right margin is much smaller. If I open the article in landscape mode first, rotate it to portrait, then rotate it back to landscape, the margins become equal.
Screenshot from the user: