If you've watched a lot of JJ's movies and TV shows, you will know that he often sets up so many mysteries, and usually answers next to none of them. Usually near the end of these movies, he will leave big hints to the answers, but he likes to leave things open ended. The fact that they outright said who Rey's heritage was in such an obvious manner shows that JJ was forced by Disney to reveal it outright, as JJ usually likes to reveal things in more subtle ways.
Just look at movies like 10 cloverfield lane, which the whole movie the viewer has no clue whats going on outside, until the very end where its still not fully explained what happened. He likes his secrets.
Even if they did plan to fully reveal all the secrets from the beginning, JJ would have still done his best to keep Rey's heritage a secret, which is why to many it comes off as a last minute decision.