Perhaps not a bug, but it took me by surprise -- perhaps an FAQ?

Source account = email backed up from
Target account = email restored to

Previous setup:

Target account was setup to fwd all email to the Source account and then delete (thus all email within the past 30 days was in the Trash of the Target account).

When restoring to the Target account, any email from the Source account that existed in the Target account Trash was left in the Trash, rather than being re-labeled to match the Source account's backup.