I don't currently have access to a computer so I can't copy and paste the In game message right now but it appears that Allison and the Vasilif of Astrum pre-planned the move explicitly to work around game limitations.
To clarify: The Vasilif (Ruler?) Of Astrum just stated that Allison moved the duchy to Astrum with the preconceived intent to simply secede the duchy again as a new realm as soon as she is able. This is explicitly against the game mechanic that prevents the Duke of the capital from seceding to form a new realm and it seems was planned as such so that a character can avoid in game consequences.