Traditional IRA to Roth IRA "backdoor" question

Hi,

 

I generally do the backdoor Roth IRA.  I usually do the Traditional (non-deductible) and then immediately convert to Roth within a few weeks.  I think in 2018 I forgot to do this, so did 2018 and 2019 amounts both in April of 2019.

 

So now the 2019 1099-R shows the combined amount, say $12,000.

 

Although TurboTax indicated "Good news, no taxes owed on this!" (since I checked that the entire amount was rolled over/converted to a Roth IRA), my refund lowered by ~$1,400.

 

Does this make sense? Did I do something wrong?  Is this because my 2019 contribution was only $6,000 and so somehow I got taxed on the other $6,000?

 

Any guidance you give me would be great!

 

Thanks!