The issue surrounding this matter regarding the titles is likely because of the titles that the duke's family carries. Since the duke the FL father is still alive she can't carry that title until he dies or abdicates. So what she is being called as a viscount is the duke's her father's subsidiary title that he carries with the title of duke. If the father would have had another daughter she would just be called a lady alone. It can be confusing as well because a dukes child as well can be considered a princess or a prince based upon if they are coming directly from nobility. Take for instance King Charles 2nd son Prince Harry is called the Duke of Sussex and this title can pass on to his son upon his death. His son Prince Archie of Sussex carries currently the title of Earl of Dumbarton but his parents don't use the Earl title as they want his to be raised as a commoner. So titles can get confusing and often children will carry a separate title that will merge if they don't have kids at that moment but will split if and when they do have a child. So a man can in fact be called a Duke as his main title but a Viscount as well until the subsidiary title will fall to his posterity that they are not called but still carry.
I really hope that the translators will be able to agree once and for all on what the fuck the FL's actual title is, because at the beggining she was from a ducal house but as the story goes on she is also a marquis a viscount, im not a translator but is it really that hard to stick to one title ┑(￣Д ￣)┍