Basic concepts:
- If you are a resident of state A and working in state B, you owe income tax to state B on your income from state B as a non-resident, and income tax to state A (your permanent home) on all your world-wide income.
- Your domicile is your permanent home. You will generally owe a resident tax return for all your world-wide income to the state where you are domiciled. There is no single factor that determines domicile, but some of the factors are where you actually live, where your significant social relationships are, where your children attend school, where your regular doctor and dentist are, and so on. Just moving to another place does not automatically change your domicile, you also have to take active steps to give up your previous domicile.
If we consider that September 2020 is when you formally gave up your WA domicile, then you were a MO part-year resident. You were a non-resident from March-August, and a resident from Sept-December (or whenever you surrendered your lease in WA). That means that you owe MO a part-year resident tax return, that will report all your world-wide income from September-December, and your MO-sourced income from March-August. Income you earned in January and February would not be taxed at the state level, since it is not MO-sourced and WA does not have a personal income tax. Income earned between March and the Fall that was not MO-sourced (perhaps investments or royalties) would not be taxable in MO.
