"and there is no way to say she was not suppose to receive it"

Here's a better way to understand all of this:

The payments in 2020 were Advances against a Projection; they used 2018 or 2019 tax returns. You are working on 2020 taxes, and this is the Actual Data that is used to reconcile what you were entitled to against any payments already received.

She was entitled. Date of death in 2020 has no bearing on anything.