This Voille portrait pre-dates Anna's becoming the Tsar's mistress in 1796 and her marriage to Pavel Gagarin in 1800.
From Wikimedia.
The fundamentals of equestrian dress have not changed. Anna wears a masculine jacket with a buttoned cummerbund (called a waist band in the keywords below) and a hat adorned with a feminine bow for her 1797 Voille portrait.
Keywords: 1792, Voille, Anna Lopukhina, Lopukhin family, Gagarin family, Princess, Russian, mistress, Tsar Paul I, long curly coiffure, hat, bows, high neckline, fichu, jacket, vee neckline, buttons, long close sleeves, waist band