It would probably be a far more settled matter, since inheritance was so important. Or not. Seeing as how wars were literally fought over the matter.
To use an example in current force, the Duke of Cornwall gets the estate of anyone in Cornwall who dies without a will and who doesn't have any surviving relatives who can claim. That means in the 21st century, Prince Charles has been funding his charities with money given to him by feudal right (luckily, modern records make it a lot easier to track down surviving relatives who aren't deliberately trying to live off-grid).
See This link on Bona Vacantia - actually, that says this came into force in 1230, so in normal Ars Magica there's probably no end of a squabble in England if you die without an heir, but King Henry will introduce a law to line his pockets.
"Excuse me? I'm here on behalf of his majesty, apparently this tower's owner has been reported dead, and I'm asking if they have any valid heirs. You wouldn't try to be defrauding the King of property and squatting on his lands?"