- Grad school in NYC - not a US Citizen/Permanent Resident, in F-1 status for ~1.5 years by end of 2017
- Internship in Seattle, WA for 2.25 months (Summer 2017)
- Return job offer in Seattle, WA
- Signing Bonus paid ~October 2017, while I was in NYC
- Only withholding on my W-2 is Federal
- Sprintax suggests I should pay NY State taxes, although this seems wrong, since the income(s) were not sourced in NY state
Even though you are considered a nonresident alien under the Substantial Presence Test for federal income tax purposes, if you resided in NYC for more than 183 days during 2017, the NY department of revenue will consider you to be a resident of NY/NYC for the entire 2017 calendar tax year.
As a resident of NY/NYC, you are required to file a resident NY/NYC income tax return which would include all income from all source include income earned during a temporary internship in WA and any signing bonus related. This is true even if your income was earned in another state.
Usually you would pay nonresident state income taxes related to your out of state sourced income and get a tax credit on your NY resident state income tax return but since WA has no state income taxes, then you would only need to income this income on your NY resident state income tax return.
For more information on the NY state filing requirements, please click on this link - https://www.tax.ny.gov/pit/file/do_i_need_to_file.htm
Additionally, what about the "Permanent place of abode" test? Am I considered to have a permanent place of abode in NY state, despite being a foreign national?
Since you are a grad student, the special NY rules related to undergraduates would not apply. Instead you would have to look at the basic rules for determining NY residency.
You had a place to stay (abode) in NY for at least 11 months and you were actually in NY for at least 6 months. Maintaining an abode would be the same for both a US citizen and a foreign national as it related to the NY residency test.