Absent nasal bone low risk NIPT

We had an absent nasal bone in 12w scan. We did NIPT which came back low risk.

But now for the 20w scan we had a intracardiac echogenic foci in left ventricle of the heart. NT was 5.7.

Our OBGYN asked us to do amnio now. We have been through hell during the waiting period for NIPT. Now we have to do it again for another 2 weeks for amnio. I am wondering if it will be a false negative NIPT. What are the chances of a false negative NIPT??