My company is over 50 employees. We typically post all job openings internally to meet our affirmative action requirements. We have an employee that is going to go on maternity leave and return part time rather than full time. Would you think that would require a job posting? Their duties will be the same just less hours.

Job postings are only for positions that need to be filled. If you're not filling the hours that this person will not be working, then you have nothing to post.