A car with a stolen part does not, itself, become a stolen car, because there’s a clear division between the part and the car itself. The only thing the police would want to seize as stolen property from you is the stolen part—they’d get you to take it out of your car, and be on their way.

Cryptocurrency is less like car parts, and more like gold: putting the crypto through a mixer is like melting some (unknowingly) stolen gold jewellery into a batch of non-stolen gold, forming new gold bars, and then selling those. Is the whole batch of gold now “tainted” by the stolen gold? Does it all need to be seized as stolen property? If not, then what does need to be seized? An equal shaving of gold from each buyer’s new bar?

If it were possible to figure out which mixer recipient was the original jewellery thief, you could just seize their mixed assets, since those assets should be equal in value to the stolen input assets. But the whole point of a mixer is to destroy that linkage, such that you have no idea who any of the recipients are.