The 8880 is a credit not income. The credit, if any, on the 8880 line 12, goes on the 1040 schedule 3 line 51. What makes you think otherwise?
Or did the IRS simply disallow the credit? The credit is only for voluntary contributions, so if your contributions to the 403(b) are mandatory they cannot be considered for the credit. State pick-up contributions under section 414(h)(2) are also ineligible for consideration since these are treated as employer contributions, not voluntary contributions made by the employee.