HSA; looks like my HSA is being taxed at a 11.9% rate!

Expenditures from HSAs are not taxable income, absent a situation with an over-contribution, which I don't have.  All of the worksheets look good--the worksheets show $0 in taxes owed on the 2020 distribution (which was used for ordinary medical expenses--which is also essential to preserving the not-taxable-income).  YET:  to test this, I ran my taxes without the HSA distribution, and when I do that, my refund goes up a few hundred dollars (in an amount equal to 11.9% of the HSA distribution).  Because my refund goes down when I include the HSA distribution and form 8889, this means that TurboTax is calculating and including a tax that I'm paying on the HSA distribution--which is incorrect.  So what am I missing?