I wrote an article concerning a fix in SP1 that was not on the original release notes. Since posting my article Microsoft has documented the issue on TechNet. The KB clearly states that the resolution is to install SP1 and also offers a workaround for those who have not installed SP1 or who have yet to migrate users to 2013. I will state that the workaround in the technet and annotated in my previous post does not work on all folders. I have first hand knowledge of some MRM 1.0 folders such as Junk Mail and Deleted items that still fail to delete when retention tags clearly show the item is expired. The image below is a view of an item currently in my deleted items folder.
While the workaround to remove MRM 1.0 policies prior to migration is a recommended step, the full resolution will still require an SP1 install to your environment.