I am asking: If you scrap a piece of Titan Residuum Gear, purchased with Titan Residuum, do you get back the full amount of Titan Residuum used for the purchase?
No. Because if you could do that, you could just buy and scrap over and over until you got whatever you considered best in slot-- you could even do that as a sort of reforging exploit. And Blizzard isn’t going to let you do that, because they aren’t that stupid or insane.
You’ll get whatever TRes you would have got from scrapping any other Azerite gear of the same Ilvl.