Mystic shield doesn’t say it makes the weapon nonmagical, just that it negates the enhancement bonuses and special properties for attacks against the target. It’s still magical, so starmantle can do nothing to it.
Actually, for that matter, antimagic field still doesn’t let starmantle destroy magic weapons; they are still magic weapons, even if their magic isn’t functioning.
Also, just as an aside: starmantle is a really problematic spell. In fact, most of Book of Exalted Deeds is rather problematic. I advise caution when dealing with its material. It’s got a handful of excellent options (Intuitive Attack, Nymph’s Kiss, and soulfire are some of my favorite options in the system), but much of it is just bad, or bad for the game (like starmantle, or any of Vows).