You will file two state returns for 2021: a part-year resident return for DC, and a resident return for MD. Here is MD's rule:
"If you are a resident of Washington, D.C. or Virginia who works in Maryland, and you do not
live in Maryland for more than six months during the calendar year, you are exempt from
Maryland state income tax withholding on wages and salaries earned in Maryland. If you do
live in Maryland for more than six months, you are considered a statutory resident and will
need to file a resident income tax return with Maryland."
https://www.marylandtaxes.gov/forms/Personal_Tax_Tips/tip56.pdf
For 2022, file MD Form MW507 with your employer and check Box 4, indicating that you are a DC resident and thus exempt from MD withholding. Here's a link to the form:
https://www.marylandtaxes.gov/forms/current_forms/mw507.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
