The new FPL numbers are released each year around the end of January. At that point, states start using the new numbers immediately to determine eligibility for Medicaid and CHIP. But the exchanges continue to use the prior year's FPL numbers to determine subsidy eligibility for all plans effective that year. They don't switch to using the current year's FPL numbers until open enrollment begins in November, for coverage effective the following January.
So subsidies for a plan with an effective date in August 2020 (for someone using a qualifying event to enroll outside of open enrollment) will be based on 2019 FPL numbers, while a person enrolling in Medicaid in August 2020 will be determined eligible based on 2020 FPL numbers.