If you receive funds for expenses and you are not required to account for those expenses, they must be included in your taxable income. They must be included in your W-2 wages, and the reimbursements are subject to Social Security tax withholding, Medicare tax withholding, and income tax withholding. Your employer is not allowed to issue a 1099-NEC for these reimbursements. Everything of value that you receive from your employer as compensation for your work must be included in your W-2 wages.
If your employer is not reporting the reimbursements to the federal government as wages, then your employer is cheating the government. If your employer reports the reimbursement as taxable income on a 1099, your employer is cheating you.
Your employer can reimburse you for legitimate expenses tax free if you are required to prove your expenses. Your employer may give you an advance on your expenses, as long as that advance is reconciled in a timely manner with proof of your actual expenses.
