Claiming the SEHI deduction for the portion of your health-insurance premiums not covered by PTC would decrease your AGI and could increase the amount of PTC for which you are eligible. That's why TurboTax needs to do the iterative calculation. Without knowing your AGI without any SEHI deduction, your filing status, the amount paid for health insurance, the amount of the monthly second lowest silver plan premium and the amount of Advance PTC you receive, it's impossible to know the best approach to take.
Getting a refund of $4,400 more if you don't take the amount of SEHI for which you are eligible after factoring in PTCs suggests that you are entering something incorrectly. Since taking an SEHI deduction lowers your AGI, so it's hard to imagine how not taking the SEHI would produce a greater refund than taking a combination of an SEHI deduction and PTC, unless, perhaps, your AGI is really low and you receive no Advance PTCs. However, I don't see how your AGI could be that low with $50,000 of income.
