The sixteenth and seventeenth century villanella and villanelle were loosely structured songs with no fixed rule, and Jean Passerat's 1606 poem Villanelle, the example usually cited as the form's origin, was written before any nineteen-line rule existed. The strict fixed form used today was codified only in the later nineteenth century, by French poets and theorists, then read back onto Passerat's poem and the form's earlier history. Whether the villanelle should be described as a form with a continuous history running back to the sixteenth century, or as a nineteenth-century invention that adopted one old poem as its founding example after the fact, is disputed among historians of poetic form.
What would resolve this No new evidence is expected to settle it; the dispute is about which of two honest descriptions, continuous tradition or later invention retrofitted onto an old poem, better fits an already-known documentary record, rather than a fact-finding gap a future discovery would close.
OpenHistory of French and English poetic formThe Princeton Encyclopedia of Poetry and Poetics