BMR backup consists of two parts:
1. The Client's configuration information.
2. The Data that gets backed-up owing to the "ALL_LOCAL_DRIVES" backup selection.
Of these, the configuration information does not expire, it remains there in the BMR Database until deleted.
But the second part, i.e. the data of the system, is handled by NBU by its usual expiry methods.
But for a successful restore, configuration information as well as the data is needed and in your case, the latter was not available since it had expired, hence the error in PTR.
Talking about connections between client's configuration information and data backup-images, have you checked out the PIT-Restore functionality of BMR? Its a useful feature if you want to restore to a configuration that is not "current" but rather some old one and you are fortunate enough to have a full-backup image for the same.
HTH,
Amol.