This happened to me a couple days ago with the Abyssal High Dragon. Got down to 0 health and I'm still attacking away until I realize that it's just sitting there doing nothing and can't be looted. Manually disengaged and tried re-attacking; called and got on and off my mount; tried wandering away a bit and nothing changed. Finally walked back to the nearby camp and rested, came back and the entire fight restarted (including the dragon flying in and busting open that little cave). Ended up dying that time, loaded a manual save I had made before placing the last bait and managed to kill it with no issues the third time.
Playing on PC. I'm not 100% sure who was in my party. Definitely Iron Bull, and I think the other two were Dorian and Vivienne. Dorian did NOT have Spirit Mark or whatever it's called (he's not allowed to have that ability anymore, lol). My inquisitor was Archer Assassin. Maybe Mark of Death glitched it out, although I don't remember if I used or triggered it toward the end of the fight.