I don’t know why or if it’s an addon or an actual bug, but my Shadowform will randomly become lighter in tone, like the glyphed version back in the day before Legion. I’ve seen it happen to both me and other Shadow Priests. Pressing Esc will usually shift it back to the standard dark colouration.
can confirm this has been happening to me as well, but can’t seem to find a pattern or reliable way to fix it. I have only noticed that it usually fixes when I hearth or take a zeppelin while in shadowform