There is no bug. If the taxable amount of an IRA distribution is less than the full gross amount due to some portion being rolled over, contributed to charity as a qualified charitable deduction, transferred to an HSA or a portion is nontaxable due to having basis in nondeductible traditional IRA contributions, the gross amount is required to be included on line 4a but only the taxable amount is included on line 4b. Only the taxable amount of this income on line 4b adds to your AGI and affects the rest of your tax return.
