I am using picketlink 2.5.1 (module manually included in AS 7.1.1 from EAP 6.1.0) and I am facing this issue too.
We found an issue with the HTTP Redirect Binding, which as preventing the SP to restore the original request. When using POST you're able to get the original request restored.
We've sent a fix this week. We'll be releasing CR1 on Monday. I've deployed a fresh SNAPSHOT version to JBoss Nexus. Wondering if you can try it out ?
Is this return URL issue fixed in Picketlink 2.1.10?