Why Is This Game On 2 Discs?

I just got my copy on Xbox Series X and it's on 2 discs. The first disc only has like 1.5GB of data on it, and then there's a 28GB update to download. When it asked for Disc 2, I put it in, but it just immediately asked me for Disc 1 again. It looks like it's fully installed, but I don't understand why Disc 2 exists?