Since the IRS calculated amount is exactly half of what the worksheet for line 5 has calculated, the IRS may be reducing your credit because they have information that the IRS sent you the Advance Child Tax Credit in the months of July thru December 2021. The Advance CTC was $250 for each child age 6 thru 18 per month.
Check your bank statements for July thru December 2021 for any payments from the IRS :
“IRS TREAS 310 CHILDCTC”
Check your tax account to see if there are payments listed - https://www.irs.gov/payments/your-online-account
