This IS an error in how TurboTax calculates form 8915-e. If you read the instructions for that form carefully, they state that you are to use the amount in box 1 of the 1099-R. They explicitly say box 1 in the instructions. TurboTax can't handle an empty box 2, but they should be able to.
Update: I just loaded TurboTax again and they have fixed the error. Now if you have a blank box 2a on your 1099-R the system correctly calculates your distribution from the data in box 1.
