So you're saying you filed your first I-539, a Change of Status to B2, and it was approved 9/2/2021 with an I-797A with an attached I-94 showing B2 status expiring 9/3/2021? And you filed a second I-539, a B2 Extension of Stay, and it was received by USCIS on 9/4/2021? Didn't it take at least a few days from approval for you to receive it in the mail? and a few days for your new mail to reach USCIS?
In any case, if my understanding of the dates is correct, then your second I-539 (B2 EOS) was filed after your B2 status had expired. Therefore, it is a "late" EOS, and USCIS generally does not accept late EOS/COS without a good cause. Since it was "late", it does not protect you from accruing unlawful presence while it is pending, unless it is approved. So, if it happens to be accepted and approved, you will not be considered to have accrued unlawful presence; but if it is denied, you will be considered to have accrued unlawful presence since the I-94 expiration.