I am using Pro series Pro 2024, and I am not seeing what you are describing. When I enter a value in 12c (investment interest income for property debt held for investment only) it does not link it to other deductions on page 1-2 line 20 (you said line 26, but that is for Amount owed). The program properly put it on the shareholders K1 line 12 code H. I suggest you do all the normal troubleshooting steps including updates.