That wasn't quite what I was suggesting. The key part I added was to have the marker entity teleported from spawn to a chunk that can never be loaded, at a coordinates relative to the players death.
Teleporting entities and then targeting them seems to work as follows:
Case 1. If origin is loaded and destination is loaded; entity can be targeted until destination unloads
Case 2. If origin is loaded and destination is unloaded; entity can be targeted until origin unloads UNLESS destination becomes loaded before origin unloads. Then entity can be targeted until destination unloads.
With the way you have it you're getting the first case whether or not you summon the marker in spawn, so you lose the entity eventually.
What I did was to run
/execute @a[score_HasDied_min=1] ~ ~ ~ /teleport @e[name=Back,tag=AddID] ~+210000 ~ ~
on an entity in spawn
Which forces the second case, and since the spawn chunks don't unload you can target it indefinitely. The value of 210,000 comes from the world boarder, which I set at 200,000. That way the chunk the marker is in can never be loaded by players, which would prevent you targeting it once that chunk unloads again.
Then you can just do
/execute @e[name=Back,tag=AddID] ~ ~ ~ /teleport @a[score_HasDied_min=1] ~-210000 ~ ~
