Using a texture pack, transparency on armor on an armor stand is only transparent when a player head is mounted on a neighboring armor stand. Otherwise it is completely opaque. Moving into the area that caused the potentially related issue (see MC-119430) also caused the transparency to go opaque, even when the aforementioned head was in place. See attached screenshots. The armor is not enchanted. The structure is a floating box of one-block thick walls. I can recreate the issue consistently in that place.