The 2nd $1,200 EIP arrived between the time we filed our original 2020 tax return and our 2020 amended return. Why our did our tax increase $1,200 on the amended return?

We had $0 tax liability on our original 2020 return. We then received the $1,200 EIP. The amended return still has $0 tax liability, but now we suddenly must pay $1,200.