I was reading and I think 18 USC 1957 may be a problem.
> Whoever, in any of the circumstances set forth in subsection (d), knowingly engages or attempts to engage in a monetary transaction in criminally derived property of a value greater than $10,000 and is derived from specified unlawful activity, shall be punished as provided in subsection (b).
The withdrawal from Tornado Cash after the sanctions were imposed is the specified unlawful activity (IEEPA violation).
Even if the withdrawal itself was legal (the person who withdrew the money was not a US person), the government could argue that he should have known the funds could have came from some crime, i.e:
> In a prosecution for an offense under this section, the Government is not required to prove the defendant knew that the offense from which the criminally derived property was derived was specified unlawful activity.