You follow the process to understand what happened and when it is considered to have happened.

"Escrow" typically reflects funds or other assets on hold (as in trust or secured; time restricted for vesting, for instance), but the financial transaction might have been fully reported in 2017 (if these relate). Just not fully dispersed. The funds or asset being dispersed in 2021 would be like the final banking (if these relate). I can sell you a house and you and I agree you will put 1/3 of the sale price in escrow, not to be paid to me until the 3 year mark, for instance; we want the release of the remainder of the funds to be based on some contingency such as, you agreed I could live there for 3 more years and I need to move out to get the rest of the money. But now I live in a house that I don't own.

If these were not funds but shares (you only stated "an acquisition of TP’s employer" and not ESOP or units or stock options or some other equity or cash-equivalent vehicle), then it is likely the 2017 reporting would be basis (thinking of the example of a 5-year vesting requirement). The release of the investment/equity from escrow is just that: released from being held, and now at the discretion of the taxpayer to hold or sell.

There doesn't have to be a 1099-B. Since there is, you evaluate what was sold and how that activity relates to the other activities. Perhaps this taxpayer then disposed of whatever got released, as Sold, triggering the 1099B for 2021.

"The escrow final release in December, 2021 was not on a W-2 as TP’s employment was in overseas."

Does it seem likely that the employer knows to report what needed to be reported on W2, though? Or is that part of the question, that you don't trust the W2 is correct, anyway?

Example: The acquisition of this company involved a liability to existing employees: an employee stock option or award was triggered, that was time-restricted until they have worked there 5 years (or 5 years after the sale, to keep good staff from leaving), and might not include a 1099-B at that point, unless the employer optionally utilizes an outside broker as required or requested to handle a sale or for fractional shares that cannot be distributed or allocated. That's just an example (very much made up), but some people look at a 1099-B without understanding why it is issued, or assume one not being issued has some other meaning that is not reflective of why one does not need to be issued. And in this example, the W2 amount would be years earlier, depending on if this is restricted stock, restricted stock units, when the election is made to price it for basis, etc.

Possible process:

W2 for FMV when vesting occurred (establishes basis in 2017); then there is the time-restriction completion; then there is optional disposal through sale, triggering a 1099-B, in 2021.

It helps if the employee knows what they participated in: Restricted Stock Units, awards, ESOP, profit share bonus from the sale/acquisition, stock purchase plans, company stock savings, etc.

I learn a lot from articles; for instance:

https://www.investopedia.com/articles/tax/09/restricted-stock-tax.asp

https://carta.com/blog/breaking-down-rsas-and-rsus/