I figured out what was wrong by reading the NJ-1040 instructions from the NJ website.
The total on line 29 was beneath the filing threshold. The instructions state in that case "Do not complete lines 30 through 49. Continue completing the return with line 50."
That explains why line 31 didn't get populated with the medical expense.
Could TurboTax have better informed me of this? I don't know. Maybe it could've told me the medical expenses weren't being used because of the filing threshold.
