It felt like they weren't taking her question seriously, Naruto was looking at her like she's out of her mind and Sasuke answered "did something happen?", she then started ranting about Sasuke not being there with them.
It seemed like even Sarada was not serious about that question as she still referred to Sakura as "mama" when Naruto comforted her, the real issue was Sasuke's absence, which was explained in the flashback.
It's probably a mix of both, we still have the missing birth records issue and I don't think it was only because of the mission, so it seems they are hiding something about her birth. It is also bad writing because all off-panel statements say she is Sakura's daughter, and this chapter also implied it, but Kishi probably didn't give us a clear answer because he wants people to keep guessing for a little longer and possibly deal with it more dramatically (Sakura is heading towards them). Or he will keep it like this, and the story will go on with Sakura portrayed as her mother which should be obvious, but this way he gave Karin fans would have something to dream about so they won't get too upset. :lol