(Bloomberg) -- When an Amazon.com Inc. delivery driver drops a package at someone’s home, the customer can give them a thumbs-up or a thumbs-down—feedback the company uses to help determine how much to pay the driver’s employer and whether they keep their job.

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In doing so, Amazon is trusting the customer to provide an honest rating. But what if the person is biased against the driver?

That possibility is a matter of growing concern for Amazon delivery contractors who employ Black, Latino and Asian drivers. Time and again, they say, their employees of color get worse customer feedback than their White counterparts. Because the phenomenon affects some of their most productive employees, the delivery firm owners suspect racial bias is to blame.

The delivery contractors say they’ve raised their concerns to multiple Amazon managers. The topic has also been discussed on Ignite, an online forum Amazon set up for delivery contractors to swap advice and discuss challenges. Amazon personnel monitor the forum and sometimes participate but have never engaged in the conversations about possible racial bias, according to the contractors. Nor has the company taken steps to address the issue, they said.

In interviews, eight current and former Amazon delivery contractors operating in Los Angeles, Seattle, Georgia, Northern California and the Northeast all described the same pattern: lower ratings for drivers of color, especially when deployed to neighborhoods where their race or ethnicity stand out. The contractors’ suspicions dovetail with decades of academic research documenting how racial, gender and age bias all influence customer impressions of service workers, from waiters to taxi drivers. Companies have been accused for years of doing too little to prevent bias from affecting customer feedback but are harvesting more of this kind of data all the time.