If you have already applied Exchange 2013 RTM CU2 (build 712.22) then you should read this post from Ross Smith. Basically you need to upgrade to new version of CU2 (build 712.24).
An issue was identified in public folder permissions being lost if the public folder mailbox is moved between databases on servers running build 712.22 so a new build has been released to include a fix for this.
Unfortunately any security updates for CU2 will depend on build 712.24 so you will need to apply this update even if you are not using public folders.
Direct link to the update: KB2859928