Since PNP is based on sponsorship, if you are already working for the employer on a temporary permit when you apply for PNP, you must stay with that employer until you have your PR.
When you are not already in Canada and use an AEO to apply for PNP and PR, as long as you are not applying for a temporary work permit for another employer at the same time, your application will go through. When you get your PR, you do not have to work for the employer who gave you the AEO, you can work anywhere. The employer who gave you the AEO is also not obligated to hire you after you get PR. He may have filled the position in the meantime and not need you anymore.