Because they were used as supplicats or gorgon. Those were not harpies. It was the same character model but different characteristics made them basically a "new enemy" type. A reskin if you will. Praetorians had sheilds. So I'm only going out on a limb here that knights or the swordbearer will have it as well.
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Praetorians are Minotaurs, so they have shields. There's actual Harpies during the Templar fight. None of them have shields. Therefore, we can assume no enemies that receive shields in Nightfalls will have shields in hard raids.
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I suppose. They will probably keep them as majors and not ultras.