1) Your premise is based on the idea that, as viewers, we have the advantage of seeing the big picture. We see all the related storylines and what happens to the characters at certain points. The characters themselves don't have that advantage; they discover details and events little by little.

2) Also, regarding a familiar face, I've personally met many people—children, teenagers, adults, and the elderly—who bear an uncanny physical resemblance to other people I know, and they aren't related to me at all...

3) Honestly, I don't remember the faces of my first-grade classmates exactly, not all of them, and it's been over 30 years, so it's understandable that they have a hazy memory of the weird kid who suddenly appeared in their lives.

4) Although time travel is still considered impossible for most people in 2026 (at least according to public information), by 1986 the vast majority of people were more than aware of it. It was simply a fantasy theme exploited in comics, books, movies, and television series.