The fact that this was reported on Form 1040 line 5a/b indicates that this is not a distribution form an IRA. assuming that no part of the distribution was rolled over and that the amount included was less than that amount shown in box 2a of the Form 1099-R, this seems to suggest that you used the Simplified Method to calculate the taxable amount of the distribution. The Simplified Method only applies when you are receiving regular periodic payments from a pension, you have after-tax basis in the pension and box 2a of the Form 1099-R does not show the correct taxable amount. Once the Simplified Method is invoked in TurboTax, when TurboTax carries it's 1099-R form over to the next year it will continue to use the Simplified Method. this suggests that you might have inappropriately invoked the Simplified Method calculation in 2020.
To remove the Simplified Method calculation you would need to delete the existing 1099-R form from TurboTax and reenter it.
