This little quirk has to do with GP allowing you to use the same Single Use (as opposed to Recurring) Batch ID multiple times. If GP stored the Batch ID (BACHNMBR) in the Open or History tables after the batch is posted, as it does in the Work table prior to posting, this could prevent use of the same Batch ID again. Some (like me) might say this is a good thing because it would provide a solid audit trail between the batch and its transactions. However, the current behavior does offer more flexibility in batch naming.
At any rate, if you need to see the Batch ID for a posted batch in a Financial Transactions SmartList, add the 'Originating Source' column to your SmartList. That is where the Batch ID is stored permanently.