CANNES, France—Léa Seydoux, one of France’s most famous actors, may miss the Cannes Film Festival after testing positive for COVID-19. Seydoux has been fully vaccinated, but she tested positive while working on a film, her publicist Christine Tripicchio confirmed Saturday. She is asymptomatic and isolating at home in Paris, hoping that negative tests on consecutive days could allow her to still attend the festival in the south of France. Seydoux was set to be one of the most ubiquitous stars at Cannes this year. She stars in four films at the festival, including three that are vying for the top Palme d’Or award—an honor she shared in winning with 2013’s “Blue Is the Warmest Color.” At Cannes, Seydoux stars in Wes Anderson’s “The French Dispatch;” in Arnaud Desplechin’s “Deception;” in Bruno Dumont’s “France;” and in Ildikó Enyedi’s “The Story of My Wife.” On Saturday, Cannes organizers trumpeted the success of …