Terminating your employment at the end of 2017 does not necessarily make you not covered by that employer's retirement plan for 2018. If the employer's plan uses a fiscal year rather than a calendar year, since you made contributions for December 2017, you are covered for 2018. Your W-2 should reflect that by having box 13 Retirement plan marked; the IRS goes by the marking of this box. However, an elective deferral from your 2017 pay should have been reflected on your 2017 W-2, not on your 2018 W-2, so perhaps your employer has erred by showing this income and elective deferral on a 2018 W-2.
If the 401(k) plan uses a calendar year, you are not covered for 2018 and probably should not have received any W-2 for 2018. If the 401(k) plan uses a fiscal year rather than a calendar year, your 2018 W-2 probably should not show any dollar amounts unless you received some severance pay, but should have box 13 Retirement plan marked. I doubt that a 401(k) elective deferral would be permitted from severance pay, so it seems that there should be no elective deferral amount shown in box 12 of your W-2.
