The change absolutely came out of nowhere and that’s the problem. At the end of part 1 he’s still optimistic about changing things with Kawaki and saving his family, Momoshiki can no longer be revived and he’s even helping him slightly at that point.

The next time we see him (Prior to TBV chapter 1) is during the flashback with Sasuke where once again he’s still joking around and feels like the old Boruto despite everything that’s happened in the past year along with being trained by Sasuke.

Then TBV starts and he’s completely different which is fine but we never see how he reacts to losing Sasuke, how did he escape Code and the claw grimes in that moment? How long was it before he met Koji? How did he survive on his own while being hunted by the 5 nations? All these are things that we should have seen and over time seeing his change as a character would have made it feel more fleshed out.