That doesn't actually affect the problem at all. If the money is laundered, it's still illicit. The question is if you invest illicit money to make more money, how much is illicit? Some people (and the U.S. government) would say all of it, and probably take anything else as well because they don't like to spend time to make sure they're only taking illicit funds (and it's not in their best interest to do so).
So, say you have $10 million after laundering it through a business, and that's used to invest in housing, and that doubles, and later the government identifies all of that original $10 million as illicit because they've identified your laundering method. They are going to attempt to take everything gained, the whole $20 million, under the theory that it was all proceeds of your illicit behavior.
The problem is, what if you already had $1 million dollars free and clear from a settlement or something, and you used that as well, so $11 million was invested and $22 million resulted? Is the government going to take $20 million and leave you $2 million? Doubtful. Should they? That's an interesting question, and depending on how people view the laws involved, 4th amendment rights, and the purpose of the seizing of money, they might come down on different sides of that question.