Microsoft has evaluated this issue, and has determined that this functionality is working as designed.
There are 2 notable workarounds the customer can use to see the full name of the account:
1) The user can open the journal in Excel via the Open In Microsoft Office button in the top right of the journal form > General Journal Line Entry > Download > Open the downloaded file.
Click on the blue field just below the Main Account field and the Data Connector add-in on the right side of the file will show all the possible Main Accounts and their full names.
The user can then add data to this journal and publish it to the AX client via the Publish button on the Data Connector add-in.
2) Hover the cursor over the main accounts in the dropdown menu and a tooltip with the full name appears.