Weir’s book makes that same point about the dearth of information. It all occurred during the great plague outbreaks in Europe in the 14th and 15th centuries.
It was very interesting to read about the sources: what little objective documentary evidence existed, and how sources from a hundred years or more later, with no particular authority, became “facts” about Anne Boleyn, just because they were “old” when the next writer came along.