It's because your TA booked through Sabre (or Abacus) and the Amadeus PNR (which is also the PNR that you can see from BA or QF) only has INFO SEG on some of the flights on other airlines but not all of them. Since Sabre (or Abacus) is the primary booking, as long as you can see the "missing segment" there, and JL confirms that, you should be fine.