Update: Today we received this email which is a bit more explanatory 🙂………
Greetings,
The National Visa Center (NVC) received your inquiry and the attached I-797 Notice of Action - Approval Notice. However, we do not have a record of receiving your approved petition from U.S. Citizenship and Immigration Services (USCIS).
We will contact USCIS to determine the location of your petition. You do not need to take any action at this time. Thank you for your patience as we work with USCIS to make sure your case is moving forward.
Although the Form I-797 approval notice mentions a delay of up to 30 days for cases to be delivered to NVC from USCIS, current shipping times may be longer.
Once NVC receives this petition from USCIS and enters it into our system, all case parties will receive a notice that they can begin processing if a visa number is immediately available.