Because they did pay you the bonus in 2021 (the pay back wasn't until 2022), so it will remain on your 2021 W-2. Boxes 3 and 5 are there to calculate your FICA taxes (social security and medicare) for 2021; the W-2c corrects those taxes. The income tax change will be handled on next year's return. Your employer uses accrual based accounting - you use a cash basis method (most all individual taxpayers do) when paying taxes.
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