Here's why I can not see you getting a corrected W-2 for 2019.
The fact is, your employer paid the employer side of taxes on your income, to the IRS. (referred to as the self-employment tax by some business owners, depending on the type of business.) There is nothing that will change that fact after Dec 31, 2019. That is, unless the employer is willing to file an amended tax return for the business for 2019. For a business with a large number of employees, I don't see that happening for the benefit of just one single employee. Doubtful, but not impossible.
I would think the employer woud be able to "correct" the situation with your 2020 income somehow, so that you would get the credit for the $5250 of tax free income on your 2020 tax return, that you "should" have got credit for on the 2019 return. Hopefully it's possible to do that without affecting your $5250 tax free credit on the 2020 tax return also. Actually, it would be on the employer's tax return, not yours. If possible, the amount in box 1 on your 2020 W-2 would be reduced by $5250 for the 2019 tax year, and another $5250 for the 2020 tax year. I have no clue if that's even possible though, as a corrective measure. But maybe my bringing it up will turn on a lightbulb over someone's head with a "real" helpful response.
