because they didn't receive the paperwork prior to the first ultrasound
Did you give them the paperwork before the u/s and they didn't process it?

I recall when I was PG with Mason I was covered by singles cover and was told that I need to upgrade to family before the first u/s as if there was an abnormality it would be classed as a pre-existing condition and he would not be covered in the private hospital.
This was in the small print that I needed a magnifying glass to read but was only told to me when I asked if my bub was covered if I stayed on singles cover.