I'm not aware of a situation where the amount in box 2(a) on your Form 1099-R entry would be automatically filled in after you left it blank. It is more likely you entered the number in that field when you entered the Form 1099-R. If the amount in box 2(a) is blank, the amount of your distribution in box 1 may or may not be taxable, that would depend on the entries in box 7 and your answers to the questions in TurboTax after you entered the Form 1099-R.
To test my hypothesis, I suggest you delete the Form 1099-R and re-enter it.
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