The problem is the underlying WebView component which doesn't handle proxy settings correctly. There's also a blog post from August 21st, 2013 on this problem. Nevertheless, the browser with over 1 million installations is still available in the PlayStore and might be widely used. The developers offer an alternative solution with the Firefox Addon proxy mobile that isn't affected by this bug.
More details are available at Why you really shouldn't use Orweb anymore.
Quick-Check (to be accessed from Orweb): http://xordern.net/checkip
The ip leakage can be reproduced with at least Android 2.3.5, Android 4.1.2 and Android 4.3.