Ideas
The only time I have had this happen is when I have a sim in a career that totally goes against who they are and what they want to do. I would send her to work fully happy and everything but she hated the job and wanted to do other things with her life. I was only trying to get her an aspiration reward and then I was going to have her quit the job and do something she loved. Everything happened fast and it was torture for her and me in getting the aspiration done because she got the mental fog, burn out, and an early midlife crisis very quickly. When I finally changed her to a job more suited to her she was fine and there were no more problems.
I wanted to share this because when I saw this post I thought maybe you are having the same problem with your sim and they really just hate the job that you are making them do. While we do control things our sims do, they still have a mind of their own and my experience with this taught me that they can be VERY willful and stubborn. Most sims don't seem to care much, but there are a few that were like this for me and it only ended when I let them do what made them happy.