Why does Eren loses his equipment after being Titan ? And why Annie doesn't lose the equipment after being Titan ?
Titans do not lose their equipment when they transform. The only reason Eren lost the gear once in S1 is because he had been partially eaten, sitting in another titans stomach. Showing him without clothes is more of a creative choice implying that before he changed, he was so damaged that he didn't have clothes/gear still attached to him.
Well it wasn't explained so it's probably just a mistake in the animation.The only other reason could be it melted but, that would make no sence at all because, he was wearing clothing still.The reason i said it melting was because one of his titans abilities lights it's self on fire.Meaning it's body temperature rises until it lights on fire.But even so his clothes would have burned off if that was the reason.So it must be a minor mistake that was overlooked.