When you were issued the RSUs, that was a taxable event. The value of all 50 shares was ordinary income, and subject to federal, state, and payroll taxes just like your paycheck. That's why the value of all 50 shares was included on your W-2. The 19 shares that were withheld were done so to pay the tax on the value of all 50 shares.
When you report the sale of the 31 shares, if the cost basis on the Form 1099-B is not the same (per share) as the value that was included on your W-2 you will have to adjust your cost basis; otherwise you pay more tax than necessary.
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