I think the issue is with how the fight is set up, not the frames themself. Basically, on Heroic and below, the targets don't actually die at any point, so the frames aren't released. On Mythic, you actually kill them after they come back.
Though the motes not shown will never be shown in the boss frames, even when you kill the visible ones. Can be rather confusing when you're stuck behind a bunch of pillars and can't tell how many are still alive.