When Biff came back to the future in part 2, he vanished while getting out of the time machine (It’s on the dvd’s) Because he was erased from existence. And you are correct, time was transformed around them in the 2015s, but it was a subtle change in that area, so they didn’t notice
When Biff came back to the future in part 2, he vanished while getting out of the time machine (It’s on the dvd’s) Because he was erased from existence. And you are correct, time was transformed around them in the 2015s, but it was a subtle change in that area, so they didn’t notice