No idea why TX residents got to file on 6/15.  Also always thought TX doesn't have state tax filing.

If you want to change the due date on the letter,  Lacerte Screen 5, second tab (Client Letter) allows you to change the due date(s).  However, it would not change the due date for the purpose of late filing penalties and interest computation, but it seems you already said you knew how to force the correct calculation.

Hope this helps.