"The Child Tax Credit now goes on line 28 instead of 19. You get the remaining amount there. Which is better since you can get more on line 28 than just reducing the amount of tax on line 16 to zero."
I've found this to be the root of the underpayment penalty problem. The underpayment penalty form, 2210, didn't change in 2021 but the cells it references did. The first line of 2210 which claims to be taxes after credits. But, it pulls from form 1040 line 22, before the CTC is entered in line 28. And form 2210 doesn't include the CTC in line 3 "Other payments and refundable credits" even though that is exactly the title of form 1040 line 32. IRS instructions say to include ADDITIONAL CTC in line 3 but that is not the same thing as the standard CTC.
TL;DR Form 1040 treats the CTC as a payment, but form 2210 doesn't which means the CTC gets left out of form 2210 completely.
Seems to be a tax law problem, not a software bug.
