In a world which I had originally created in version 1.8 there were armor stands which had arms added to them, so that way they both wore armor and held items.However, when opening that world in 1.16.2 (and this problem still persists in 1.16.3) the stands' arms became invisible and were holding their leggings and boots instead of wearing them, causing both the chestplates/helmets to disappear and for the previously handheld items to vanish.