The IRS does not use the amount in box 2a if the Form 1099-R has code T in box 7, so TurboTax properly ignores anything you enter there (except in the case where code T is used to indicate a distribution from an inherited Roth IRA because you must determine the taxable amount outside of TurboTax).
For a code-T distribution from a Roth IRA, you must click the Done or Continue button on the page that lists the Forms 1099-R that you have entered, then either indicate to 2022 TurboTax that you had a Roth IRA before 2018 (is that's true) or you must enter your basis in Roth IRA contributions and conversions so that TurboTax can calculate the taxable amount on Part III of Form 8606.
