Check with the old company to see if there is an error, but it is probably correct. Many company retirement plan years are not calendar years and can span calendar years. It is entirely possible that contributions made by the company for your benefit came in the new year so the W-2 would be correct.
Also is there any entry in box 12 like a "D 19 xxx" where D is a 401(k) plan, 19 means the 2020 contributions was applied to 2019, and xxx is a dollar amount? However, employer contributions to the plan are not reported in box 12, but do cause the box 13 to be checked.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
