The law protects people in your situation. As you note, the health law limits eligibility for premium tax credits to people with incomes between 100 and 400 percent of the 2019 federal poverty level ($12,140 to $48560 for an individual).
The health law originally expanded Medicaid coverage to adults with incomes up to 138 percent of the federal poverty level, so it was thought that people with incomes below the poverty level wouldn't need access to premium tax credits. Therefore subsidies were limited to people earning from 100 to 400 percent of poverty. But those provisions were challenged in court, and, in 2012, the Supreme Court ruled that the Medicaid expansion was optional for states. So far, 25 states and the District of Columbia have expanded coverage.
However, a Treasury Department special rule addresses the potential financial repercussions for people like you. The rule says that if your state exchange determines that you're eligible for a tax credit, but your actual income at year end puts you below the poverty level, you won't have to repay any tax credit amounts that you received.
"In this situation by definition the annual income is less than the original estimate, so there won't be any obligation to repay [the premium tax credit]," Judith Solomon, a vice president for health policy at the Center on Budget and Policy Priorities, said in an email.