New York Tax Law 631 defines non-resident income attributable to "a business, trade, profession or occupation carried on in this state" as NY-source income, and therefore taxable by NY.
https://codes.findlaw.com/ny/tax-law/tax-sect-631.html
Therefore a bonus received by you in 2021 for work carried out in NY in 2020 is entirely taxable by NY, regardless of whether you file for 2021 as a non-resident or part-year resident of NY.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
