Quite simply, the lien may have been paid off at the closing of the transaction.
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You're assuming the new owner did not take subject to the lien. If he bought it without an attorney and the benefit of title insurance, that's actually a likely scenario.
I may be guessing or not licensed in your state. No atty/client relationship exists.
If there was title insurance, you would go through the title company to clear the issue. Also, it may be possible that it was filed after closing or it was paid off at closing.
This does not constitute legal advice or form an attorney-client relationship.