Regardless of one using AC21 or not, CBP officer can ask the same question and harass if they want. There is no law to begin with that defines that you have to be working for the petitioner when one is filing for AOS or any stage of GC process. GC based on employment is for "future" employment.
Ethically it makes sense to work for the employer who you will be working on a "long" term project and who wants you to get permanent residency. There is no agreement or obligation that restricts employee to reject the employment in the future or employee to not return and that in eyes of CBP officers is a FRAUD.
I have read blogs where people write that they were told to sit for 2-3 hours before they returned the stamped document and same question was asked them and it took them 2-3 hours to figure out whether to let those guys in or deport them.
On the positive note, they can not deport you because you are not working for the original petitioner.
Originally Posted by when
If one were to port using AC21 and inform USCIS about the change, why would they still harass?
Did you change employers under AC21 (485) and/or did notify USCIS?
I very much appreciate your insight
Thank you & Regards