I have the exact same problem. I received an 1099-INT form lately from a bank which does not mail the from on-time. So I started an amended return with TurboTax Deluxe 2018 (Mac). In my case, the refund monitor showed $18 on the first update screen, not $0 as it should. I checked the generated Form 1040X and found out the cause is that the box A (original amount) of line 1 on 1040X (AGI) is $150 lower on my originally filed 1040.
