Marvels Agents of S.H.I.E.L.D has been consistent over the time with the MCU, so that both storylines belong to the same universe and reality. There are numerous occasions when characters and storylines of the MCU are mentioned in the series (Ultron and the Sokovia Accords, Captain America destroying the Triskelion, Thanos, etc.) However, why does time travel behave different when they both are inside the same reality?
According to the MCU Universe, the rules are clear: "Going back in time does not change the past"
BRUCE BANNER: ...And secondly, time doesn't work that way. Changing the past doesn't change the future.
and later on
BRUCE BANNER: I don't know why everyone believes that, but that isn't true. Think about it: If you travel to the past, that past becomes your future. And your former present becomes the past. Which can't now be changed by your new future...
SCOTT LANG: So... Back To The Future's a bunch of bullshit?
However, in season 5 of Marvels Agents of S.H.I.E.L.D, changing the past was indeed possible. They travelled to the future to see a dystopian reality where Earth was destroyed by Quake, the Destroyer of Worlds. They came back to change that future, so actually when they are going in back in time, they are changing the past to a reality where that dystopian future is no longer real, so basically contradicting the arguments exposed in Endgame.
Even worse, the whole of season 7 is based on the premise that changing the past is possible by time travel.
Am I wrong or is there a better explanation to this?