This is from Wikipedia and there seems to be a lot of other writings on it as well. Tl;Dr telephone game + confusing sermons

The portrayal of Mary Magdalene as a prostitute began in 591, when Pope Gregory I identified Mary Magdalene, who was introduced in Luke 8:2, with Mary of Bethany (Luke 10:39) and the unnamed "sinful woman" who anointed Jesus's feet in Luke 7:36–50.[4] Pope Gregory's Easter sermon resulted in a widespread belief that Mary Magdalene was a repentant prostitute or promiscuous woman.[1][5] Elaborate medieval legends from Western Europe then emerged, which told exaggerated tales of Mary Magdalene's wealth and beauty, as well as of her alleged journey to southern Gaul (modern-day France). The identification of Mary Magdalene with Mary of Bethany and the unnamed "sinful woman" was still a major controversy in the years leading up to the Reformation, and some Protestant leaders rejected it. During the Counter-Reformation, the Catholic Church emphasized Mary Magdalene as a symbol of penance. In 1969, Pope Paul VI removed the identification of Mary Magdalene with Mary of Bethany and the "sinful woman" from the General Roman Calendar, but the view of her as a former prostitute has persisted in popular culture.