This is a weirdly specific question, but I'm writing a book and I need to understand royal family trees to get it right. One thing I discovered was the Nicholas Maximilianovich de Beauharnais the 4th Duke of Leuchtenberg had two legitamate sons, Nicholas Herzog von Leuchtenberg (1868-1928) and George Herzog von Leuchtenberg (1878-1929). So why did Nicholas' brother Eugene become the 5th Duke instead of Nicholas II or George?
Although the fourth Duke was married to Nadezhda Sergeevna Annenkova when his two sons were born, the marriage was not authorized by the Russian Emperor. Since the Duke was an imperial prince, he needed the Emperor's permission to marry a divorcee, which Nadezhda was. Since he did not receive it, the Duke of Leichtenberg was forced to leave Russia due to Alexander III's displeasure. This morganatic union was recognized as legal only after 11 years, after the boys had already been born.
Source: https://archive.is/20130126195832/www.istrodina.com/istochnik_articul.php3?id=53&n=50