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A code P on  a 2019 1099-R means it was a 2018 excess returned in 2019.   The excess must be reported as returned wages on an amended 2018 tax return.  Entering the code P in to the 2018 software will ask if it is a 2018 or 2019 1099-R - say 2019 and the box 1 amount will be added to the 2018 1040 line 1 as wages.

They cannot change the W-2 since the W-2 reports what happened in 2018 and the past cannot be changed.

You should have received a 2nd 2019 1099-R with the earnings in box 1 and box 2 with a code "8" in box 7 because the earnings are taxable in the year returned - 2019.

For information see IRS Pub 525 page 10
https://www.irs.gov/pub/irs-pdf/p525.pdf

Excess distributed to you.

If you take out the excess after the year of the deferral and you receive the corrective distribution by April 15 of the following year, don't include it in income again in the year you receive it. If you receive it later, you must include it in income in both the year of the deferral and the year you receive it (unless the excess deferral was a designated Roth contribution). Any income on the excess deferral taken out is taxable in the tax year in which you take it out. If you take out part of the excess deferral and the income on it, allocate the distribution proportionately between the excess deferral and the income.

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If the distribution was for a 2018 excess deferral, your Form 1099-R should have code P in box 7. If you didn't add the excess deferral amount to your wages on your 2018 tax return, you must file an amended return on Form 1040-X. If you didn't receive the distribution by April 15, 2019, you must also add it to your wages on your 2019 tax return.
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If the distribution was for the income earned on an excess deferral, your Form 1099-R should have code 8 in box 7. Add the income amount to your wages on your 2019 income tax return, regardless of when the excess deferral was made.