You will file a part-year return for Illinois and a part-year return for Massachusetts. Your income will be allocated to Illinois from 1/1/21-4/30/21 and to Massachusetts from 5/1/21-12/31/21 as you are responsible to pay the state in which your income is sourced (the state in which you worked to earn the money) since you have and continue to work remotely.
You will get a refund from Illinois for the overpayment since your employer never changed your state withholding, and you will owe Massachusetts for the period of time you earned money in that state and did not pay income tax. Please ensure your employer switches your state withholding to your current state of residence.
For more information, please see How do I file a part-year state return?
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