Simon Kinberg, co-writer and co-producer on the film, said it was additional shooting. He said they needed to add more action to the film. I guess the additional filming is there to add another action sequence as the film is a bit action light according to some sources.
I am not overly worried about the movie just yet. Maybe it's because I have faith in the director, Josh Trank. Maybe it's because I think the cast - which includes Miles Teller, Jamie Bell, Michael B. Jordan and Kate Mara - is pretty fantastic (pun intended). Maybe it's because I like having faith in underdogs only for them to prove people wrong. It's very possible the film will suck, but I'd like to believe it will actually be good.
While, at first glance, the lack of marketing might be discerning to some I take it as the studio trying to figure out what kind of movie they have first. Even if the film is more Chronicle 2 than Fantastic Four, that might be a breath of fresh air for the superhero genre. Who knows. It could actually work out.