Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Tax withheld by USA working in UK (Form 1042-S and Form 1099)
The shares are "imputed income" / payment in kind. You worked in the UK, but are you a "US Person"? If not, you should go back to payroll with this query as this income is taxable in the UK. It is important you find out on what basis they were issued. The company will have answers. Where they aquired at a discount to fair market value ? Where they purchased with a salary deduction as part of a scheme ? Where they acquired by conversion of employee stock options ?    If you sell the shares, or are paid dividends, then there will be tax withheld.