Look, I’m fine with certain minor things in the Star Wars universe not making complete logical sense. I mean, it seems like it’d be a lot easier if astromech droids just spoke English (Arabesh? whatever…) rather than bleeps and whistles, but who cares?
The movie straight up tells us that the resistance does not have a map of that region of space, so the fragment from Lor San Tekka is meaningless without context. Then R2 spontaneously reactivates and reveals he has the full map minus that one fragment. It seems to me like something that needs an explanation.
I could have sworn BB-8 put the last piece in…
He did, because there was a piece missing. Like I said, R2 “reveals he has the full map minus that one fragment.”