**HUGE SPOILERS ON AVENGERS ENDGAME ENDING**


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I think you misunderstood the ending: if it was the way you tell it, yes, it breaks its own time travel rules.

What I understood is that Captain America lived his life in the "other" past (other timeline) and then came back when he was old: I remember Bruce Banner saying something about Cap' having used its device to come back. That way, it doesn't break the rules.