So? Isn't losing medicaid an ACA qualifying event? Your income goes up, you lose the medicaid but now qualify for ACA.
That said, in my secondhand experience across multiple states I won't name, the medicaid office probably won't actually find out or do anything before 12 months are up.
Also, a few states are starting to officially enact "continuous eligibility" and intentionally not check your income for 12 months -- mostly for children[1] right now, but some states for adults[2] too.