The FPL levels used for the ACA PTC calculations are offset by a year. This is a known feature of the tax system and has been in place since the ACA was put into place.
Like many things, it is a timing issue related to when the figures are available and when the IRS and the ACA marketplaces need them to be available. In this case, the ACA marketplace needs FPL numbers for tax year 2020 in late fall of 2019 in order to properly calculate the APTC subsidies for taxpayers during open enrollment. While the IRS tax filing stuff is a whole year later, if they used the next year's (2020's) FPL numbers then the APTC calculated by the marketplace in fall 2019 wouldn't match with the Form 8962 calculations done in the tax filing process.
You can certainly file your 2020 tax returns as soon as (a) all the forms and instructions you need are finalized (the 8962 is final for 2020), (b) you have all of your data, (c) the IRS is accepting returns.
The 2020 Form 8962 instructions will use the 2019 FPL levels, and the highlighted number will remain $12,490. No need to wait for any update, because there won't be any. I would expect the 2021 Form 8962 to use the $12,760 number.
