iPhone X flash doesn’t work when using the camera when it’s cold , the flashlight app works.

First, you are not directly talking to Apple here. We are fellow Apple-product users just like you.

Second, how cold is the temperature that are you taking about? The following is the Environmental Requirements (taken from Apple Technical Specifications for the iPhone X):

Environmental Requirements

• Operating ambient temperature: 32° to 95° F (0° to 35° C)
• Nonoperating temperature: −4° to 113° F (−20° to 45° C)
• Relative humidity: 5% to 95% noncondensing
• Operating altitude: tested up to 10,000 feet (3000 m)

Finally, all mechanical/electrical equipment can "fail to operate" properly in extreme cold temperature. Batteries are one perfect example.

Here its 10+ celsius