NAB Tier 1 Floating Rate Bond - Franking Credits not posting to Tax Label 8H

To begin with, this is not an ASX listed hybrid, but a interest earning unsecured, perpetual Tier 1 floating rate bond. As a bond, income links to an interest income type account, not dividend or distribution type accounts. When entering interest income transactions from coupon payments I also enter the franking credit next to Franked Dividends and show $0.00 in the Franked Dividend field. (The interest amount populates the “Gross Interest” field correctly.)
Problem is, although the Investment Income Report shows the interest and franking credit appropriately, the franking credit does not flow through to the tax return Tax Label 8H “Excess franking offsets” where all of the other dividend franking credits appear.
How should I be accounting these franking credits in data entry?
(I am not an accountant so my head is not fully around tax return complexities.)
Any help gratefully accepted.