This happen most of the times with Chinese webtoon. First, the author pay a lot more to hire a high level artist and good editor. once the story start to sell like hot cake and it's hard for people to stop reading after going so far in chapters, author change their artist/editor to the cheaper one to cut cost and earn a bit more.
Either that's what happening, or this happened:
The author/artist have some kind of disagreement with the company, decided to leave the company, but because a unfair contract, author/artist lost their right to the story. There for, the company hire someone else to continue the story because they can't just let the money maker story to stop in the middle....
Wow umm
Why did they change the art style??? I mean, it’s good but the first art style was like a million times better than the art style they used for the end parts