The tax law change is Section 9042 of the American Rescue Plan Act, which retroactively excludes from tax up to $10,200 of unemployment benefits if your Adjusted Gross Income (AGI), not including unemployment, is less than $150,000. This change is for 2020 only. The American Rescue Plan Act was signed into law on March 11, 2021, but it took a couple of weeks after that for the tax software companies to update their software.
I suspect that the IRS might have held your amended return until they recalculated your original return for the unemployment exclusion, but that's just speculation on my part.
