Hello were you able to find out more information about this? I got the exact same letter Friday under the exact same circumstances. In the email they however did give me a NVC case number and when I look it up on ceac it says "at NVC". I called USCIS and they gave me an extremely vague/written answer that it has not arrived to them yet but would be with them for at least 180 days. They could not say why it was sent to them though. Very very confusing