After you file

For now the third time in five years, I have received a letter from the IRS asking me to mail copies of my claimed federal tax withholding by Social Security and for my IRA.  No problem, mailed copies showing exactly what I had claimed in my 2021 federal tax return.  But I thought that if IRS challenged you two times before on an issue, with no change in tax due, they weren’t suppose to question you for a third time.  Is this policy no longer in effect?  And why doesn’t the IRS already have a copy of these withholdings?