Yes, it was known for over a year. I In the summer of 1999 the first pictures were taken of WA with an unidentified woman aboard the yacht of his grandfather, prince Bernhard. First she was identified as Maxima Zimmerman. That caused som eyebrows to rise as Some people with german names in South America are (grand)children of escaped Nazi's. I remember an angry letter in a newspaper complaining that it seemed to be that the dutch RF only allowed itself to marry germans.
Later on television news and -de Volkskrant' revealed the true identity of Maxima, when she, WA and her relatives were skiing in Bariloche. Then the problem of the father came up. The prime ministers measures and Maxima's charming behavior later cooled everything down and silenced most critics.