Visual Studio and EF control model generation and re-generation. Oracle cannot set StoreGeneratedPattern itself or maintain its value after re-generation. If Microsoft provided a way for third-parties to control how these EDM attributes were set and maintained, we would. You may want to ask Microsoft when they will provide such a capability.
I assume that you have already installed the latest Visual Studio service pack and fixes related to this problem. In case you haven't, make sure to have SP 1 of Visual Studio 2010 or higher installed.
For some SP 1 users, the problem persisted and MS published the following note related to this bug: http://support.microsoft.com/kb/2561001
The hotfix for this bug is located here: https://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=37957
I can't guarantee that either of these solutions will resolve your problem, but you should try these first to ensure it eliminates known StoreGeneratedPattern issues.