If the funds were distributed to beneficiaries in 2021, then the estate's Form 1041 would include Schedules K-1 showing how much of the income they report on their 2021 returns.  If those returns have already been filed, they will need to file amended returns.  If no distributions were made, the estate owes a lot more tax than the 10% that was withheld.  Worst possible case, I suppose, is that the funds were distributed in 2022 and no one wants to return any of it.