Here's how/why I created a profile "Unknown Unknown." (It was unsourced, too.)
Through US Census records, I had a wife/mother with an unknown LNAB. We've all seen that in Census records. Later in life, a widow, she lived with her sister, also a widow. The system automatically generates a profile for the father of siblings, and since we don't know their LNAB, we get Unknown Unknown as father. No dates and no sources either. And yet, because of the two daughters, we do actually know something about him.
As to Unknown for LNAB, apparently it is now being accepted again. I've not had occasion to try, but I presume it will now work for me, too.