Recovery Rebate Credit: EIP2 Payment Received in 2021, Report on TY2020 or TY2021?

Hello Tax Experts,

  My taxpayer received the first stimulus payment in May, 2020, in the correct amount for she and her husband and their three dependent children ($3900).  EIP2, however, did not arrive until February 2021 ($3000).  Since she is a cash-basis taxpayer, filing a joint Individual return, can she ignore entering her EIP2 payment amount on Line 20 of the Recovery Rebate Worksheet, but then report it on her TY2021 tax return?  I'm sure the IRS has well-thought this out and has planned ahead for a second- and third-year stimulus "true-up" at the end of 2021.  As a cash-basis taxpayer, every other payment received on her return is reported in the year it is received (including state tax refunds).   If she had already filed her return in January, before receiving her EIP2, would she then be required to file an amended return?  Why can't the IRS come out with an authoritative ruling on this subject?

Thank you in advance for answering my question.

Magic Lou