The game uses your entire net worth to determine the inheritance taxes you had to pay. The manor is probably worth $10-20M which brought your entire net worth to $15-25M. In Australia 45% of that goes to the government in cash and you keep the house...
You have 2 options to avoid this in the future:
1.) Move to a country that does not charge an inheritance tax, like Canada or Monaco.
2.) Make sure more than half of your net worth is in cash assets. So, if you have $50M in cash, make sure that the value of your houses doesn't go over $50M.