@Illia , thank you for your answers to my questions.
(a) Have gone through the US-Ukraine tax treaty again and while there are significant differences ( from many others that I am aware of ) in articles treating students, researchers etc., there is nothing that precludes a resident/ national from "extending" tax residency . However, I do note that Ukraine also determines tax residency based on citizenship ( when there is ambiguity in tax residency). Also that Ukraine taxes its residents based on world income.
(b) Noting that for US immigration purposes, a J visa holder is allowed 30 days of stay-over at the end of contract with the sponsor ( J visa sponsoring org.)
Therefore my conclusion that there is nothing preventing your parents filing a MFJ for the whole of 2025 as long as they include /allow /recognize their world income for the entire calendar year. For 2026 they then have only 1040-NR to file covering any US sourced income.
Unless they plan to come back to US, they should consult with a tax-professional (familiar with international taxation) on how to proceed with tax deferred accounts like 401(k) or similar.
Is there more I can do for you ?
pk
