Unless he suffered horrific brain injuries, it's not the same artist.
It is, he just made a bunch of lower quality art that was supposed to be used for side content and repeatable scenes. They then used the art as the main assets for a bunch of scenes, and the artist has been sick and / or lazy about doing the better-quality pieces they used to.
So unless Nomo (the artist) has actually left the project and they've hidden that he left, the artist hasn't been switched, just done shittier drawings.