Ok I think I figured it!
I was not entering the basis correctly in my 2020 return (step 9 in 2020 return instructions https://ttlc.intuit.com/community/retirement/discussion/re-backdoor-roth-ira-conversion-multiple-yea...)
I was entering 0 while I was supposed to enter $6K. It's surprising TurboTax didn't pick it up automatically from my 2019 form 8606 (which mentions $6K in line 14 -- total basis for traditional IRAs 2019).
Now my 1040 line 4b also reads $2yyy, which is what I'd expect. Thanks @DanaB27 !
