No, the author and that artist are the same person. If they hire someone to do their job ppl like you would complain about the design change, bc you just can't copy exactly someone's style. They probably did always everything alone bc they wanted to have a full control on how everything looks and also didnt expect this to blew in fame like it did. It's clear you haven't read the notes the author published between chapters to let fans know what's going on.