[Hmmm. Doesn't that create a TIME PARADOX? Unless...]
If they're from a point earlier on the same timeline as us, and they now know information about the future, that means they should have the capability to change the future we remember and avoid...those incidents. That they didn't — and it makes no sense that they should fail to change that history when the information in question is the identity of the mastermind in the first place — suggests that either the history we remember is set in stone with no ability to be altered, that a deviation into a parallel timeline occurred on account of that new knowledge, or...
[Or, possibly the most uncomfortable possibility.]
Or that they'll lose those memories once they return to maintain the consistency of the same timeline we all share, because we don't remember them doing any differently.
[Video] - DR ENDGAME SPOILERS
[Hmmm. Doesn't that create a TIME PARADOX? Unless...]
If they're from a point earlier on the same timeline as us, and they now know information about the future, that means they should have the capability to change the future we remember and avoid...those incidents. That they didn't — and it makes no sense that they should fail to change that history when the information in question is the identity of the mastermind in the first place — suggests that either the history we remember is set in stone with no ability to be altered, that a deviation into a parallel timeline occurred on account of that new knowledge, or...
[Or, possibly the most uncomfortable possibility.]
Or that they'll lose those memories once they return to maintain the consistency of the same timeline we all share, because we don't remember them doing any differently.