TurboTax includes the amount from box 4 of the Form 1099-R on line 17 of 2019 Form 1040. Line 17 is subtracted from your overall tax liability on line 16 to determine your refund or balance due. Make sure that box 4 of TurboTax's 1099-R reflects the amount in box 4 of the Form 1099-R provided by the payer.
State tax returns operate similarly.
Your tax-bracket rate is not necessarily your marginal tax rate because of potential side effects due to the increase in AGI resulting from the additional income. It's impossible to know your marginal tax rate on any separate item of income without taking your entire tax return into account.
"an amount of the withdrawal at exactly my tax bracket is being deducted from my refund."
Given that you had 15% withheld for federal taxes, that result could easily be explained if you have some income taxed at 0% the long-term capital gains rate without the additional income from the Form 1099-R and the additional income is pushing an equal amount of the long-term capital gains from the 0% bracket to the 15% bracket. There are other possibilities, but with those other possibilities this coincidence is less likely.
