The issue with Retention reporting is cosmetic only. After extensive testing when actually running the policies they do act correctly and will not remove messages that should not be removed. The issue is with the explanation of the fields in the report. We do not correctly differentiate/explain Fully Occluded vs. Partially Occluded messages. The details page contains an aggregate count for messages effected by that policy, but this count is a distinct message ID count that can include messages that are only partially occluded.
This has been logged as bug - TF00491862