Probably an overcorrection from when people were complaining very vocally about ChatGPT being "lazy" and not providing all the code. FWIW I've seen Claude do the same thing when asked do debug something it obviously did not know how to fix it would just repeatedly refactor the same sections of code and making changes to comments.

I feel like “all the code” and “only the changes” needs to be an actual per chat option. Sometimes you want the changes sometimes you want all the code and it is annoying because it always seems to decide it’s gonna do the opposite of what you wanted… meaning another correction and thus wasted tokens and context. And even worse it pollutes your scroll back with noise.