시애틀 이변호사님,
https://www.uscis.gov/opt
에 따르면, 아래와 같이 설명되어있는데요. 그리고 많은 분들이 이렇게 알고계십니다.
If you file your STEM OPT extension application on time and your OPT period expires while your extension application is pending, we will automatically extend your employment authorization for 180 days. This automatic 180-day extension ceases once USCIS adjudicates your STEM OPT extension application.