"They received lower stimulus payment in round one and lower in round two because of income over the threshold."

But that doesn't matter; that is the Advanced Payment as a Projection against 2020 Actuals. Even if they had not divorced, and 2020 had been lower reported income, the 2020 tax return is the reconciliation between what was paid out already and what they are entitled to get.

They didn't get lower or higher. They just got money as a Projection. Now it is time to report reality for 2020. And allocating what they got is correct; how they split is none of your concern.