If you were covered for one day in 2018 that is what the IRS uses. It goes by when you are paid, not when the work was actually done.
I fully understand what you are saying.
You would have to convince the employer to issue a new W-2 (corrected one) and have the retirement box not checked on it otherwise IRS will deny the IRA contribution.
"my ex-employer withheld and contributed one last 401k contribution in the first week of January 2018"
