My original reply inadvertently omitted the word "not" from the first sentence, so I fixed that. It's not clear to me why any annuity that was annuitized would not be partially nontaxable unless all of the investment in the contract had already been paid out, resulting in all subsequent distributions being fully taxable. The is a provision in section 72 of the tax code that describes partial annuitization that might cause the contract to be treated as two separate annuities, so it's possible that payment of a GMWB might be considered to be from a separate contract that is not being paid out as an annuity and has no investment in the contract. Also, if there is a loan treated as a distribution, that amount is treated as an amount not treated as an annuity, but I don't see how a GMWB would have anything to do with a loan.
Box 2a should only be blank if box 2b Taxable amount not determined is marked.
You might read section 72 to see if anything stands out, but it's a complicated section of the tax code:
https://www.law.cornell.edu/uscode/text/26/72
