Hamilton Beach 5 qt Slow Cooker Bad taste and smell
It sounds like the cooker malfunctioned. An electrical fault inside the unit may have caused a short that caused some component to burn. Various electrical parts can give off strong acrid odors when this occurs. It's possible that the nature of the fault was such that its circuit only came into play when you switched the unit to "warm".

However, assuming it's a typical slow cooker with a ceramic bowl and glass cover, I don't know how the taste would be transferred inside to your cooked food. Perhaps the odor was so strong and held in the air such that it affected your taste buds.

In any case, you are wise to return the unit. Whatever happened to yours is likely an unusual occurrence. Hopefully it won't happen again to its replacement.