This is by design, The details in the This Posting section relate to the next posting that you are about to make, not any previous postings. None of those details are saved, you just enter them before you post. To do this:
From the Toolbar click Nominal Ledger.
Click Journals then click RMB to Create Record.
Enter the Template Reference and Header details.
Click Apply followed by the Journals tab.
Enter Journal details.
Click on the Header details to enter The Posting details.
If a Number Of Periods is entered this will create the journal to those periods.
To see the values that were used to post previous journals, please look on the Posting History tab.