OPT STEM extension for contractors

I am a Mechanical Engineer on my first year of optional practical training. I would like to apply for the STEM extension and have found a company that would like to hire me as a contractor. The staffing agency who I have been in contact with said that since the law changed in April 2018 it is not possible for me to apply for the STEM extension as an on-site contractor. Does anybody have any experience with the new laws and STEM extension as a contractor?