Have two children under age 5, received advanced payments, made under $40k. I entered IRS letter 6419, federal refund went down by 3,600. Is this an error with Turbotax?

Nothing is wrong---the way the software shows it to you is just confusing you.

Federal>Deductions and Credits>You and Your Family>Child Tax Credit

The IRS is sending out letter 6419 to you.  It will show the amount of advance child tax credit that you received during 2021.  Enter the information from that letter carefully.   The remaining amount of CTC that you can receive will show up on line 28 of your 2021 Form 1040.

PREVIEW 1040

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.

NOTE:   The CTC is indeed a “credit” that can be applied toward any tax liability that you would otherwise have to pay as “tax due”  to the IRS.

And…..Ignore that “refund monitor.”  The software STARTS OFF by giving you the full amount of CTC.   When you enter the amounts from your letter, the software reconciles the amount you received with the amount you can still get IF you are eligible to get the other half.     You are NOT losing any of the credit; you are not being taxed on the credit.  Don’t get confused by the reconciled amount.   You got some of it already—-now you get the rest.

 If you received the advance payment and are NOT eligible to get the other half, then it will not show up on line 28.  (Such as in some cases where the wrong parent got the advance payments, etc.)

That “refund monitor” should not be taken seriously until every last morsel of your data has been entered.