Bug reported in "My Oracle Support" :
Note 1321896.1 - 11.1.1.5.0 Regression: Rollback Button With Property immediate="true" Causes Replacing Existing Rows With Current One
Check Michael Koniotakis' post (Jdeveloper 11.1.1.5 severe bug.) for more details. Also check OTN post to see when it was first mentioned.