Are the missing parents shown with the usual blue boxes with pale blue links to Add Father and Add Mother?
It sounds like you have added that
Child to a
Family record with
no Parents in addition to their correct
Family with correct
Parents.
In the
Child's
Property Box click on the
All tab and there should be two
Parents family entries.
The first one will say
...of (parents not known).
Click on the small white square next to the icon to expand the details.
Does it just list
Father and
Mother < click here to insert link (...) > possibly with
Siblings like this:

- UnwantedParents.png (13.99 KiB) Viewed 2872 times
Select that blue
Parents family entry on left and hit the
Delete key or use
Edit > Delete.
Then
OK the warning.