2018 Form 1040

Taxpayer has a Form 1099-R with an amount in Box 1 (gross distr.) and a different amount in box 2a (taxable amt.) The box 2b "taxable amount not determined" is NOT checked.

I have done this millions of times. Okay, maybe only thousands.

Normally, the gross amount goes on line 4a and the taxable amount goes on line 4b, if they're different.

For whatever reason, just for this one particular 1099-R, ProSeries is making the full amount (box 1) taxable.

Does anyone know why?

FYI, it's box 7 code is 7, regular distribution. Not that that makes a difference, because even if it were an early withdrawal, that wouldn't change the amount taxable, just the amount that would be liable for the additional 10% penalty. 

Do I need to check a box somewhere? I've never had to anywhere else, just put the correct amounts in boxes 1 and 2a as they are reported.