"The attorneys are claiming that its a mistake from USCIS and they did file for an extension of stay in I129"
First, just because the attorneys filed for an extension of stay doesn't mean USCIS will grant it. They may not give an I94 if there is unauthorized stay, etc. Was that true in your case?
In very rare cases does USCIS make the mistake of sending 797 without I94. The best course of action would be for your attorney to call and get it corrected. If there is a case number, I think it would be OK to stay and continue working but the moment they deny, you have to leave the country and go for stamping.