We upgraded the extensibility of the posting feature to use abstract classes instead of interfaces to allow more flexibility when adding more members.
IPostingProvider is now deprecated in favor of PostingProvider abstract class.
To support the legacy plugins that are still using the old interface, we created a middle adapter layer. It's possbile that you are hitting a bug in the adapter layer.
For now, my suggestion would be to upgrade your plugin to take advantage of the abstract classes and see if the problem still persists.
George