It seems that the plan failed to prepare the Form 1099-R correctly despite actually knowing the taxable amount. (Box 2b Taxable amount not determined is for indicating that the plan does not have the necessary details to do the calculation, not that they are ignorant of how to prepare form 1099-R.)
In this case, they should have entered in box 2a the difference between the amounts in boxes 1 and 5 and not marked Box 2b Taxable amount not determined. Clearly, if they were able to calculate the 10% default withholding of $92.41 on the taxable amount, they knew that the taxable amount was $924.10 (give or take a few cents) and should have put that number in box 2a. IF you enter it in box 2a, TurboTax will include only the $924.10 on line 5b of Form 1099-R. The entire gross amount will be on line 5a.
