The IRS has not said how it will handle catch up payments and adjustments. The only thing is has said is to file a 2020 return so it has updated information.
For example, in addressing the issue of an additional dependent on the 2020 return that was not on the 2019 return, all the IRS says is: "If your third payment does not include a payment for your qualified dependent who did not receive a third payment, you must file a 2020 tax return to be considered for an additional third payment even if you don't normally file."