If you’re experiencing a backlight issue with your MacBook Pro (16-inch) 2019 display, there are several potential causes and troubleshooting steps you can take:

1. **Brightness settings**: Ensure that the brightness settings on your MacBook Pro are correctly adjusted. Sometimes, the backlight may appear to be malfunctioning when it’s simply set to a very low brightness level.

2. **Restart your MacBook**: A simple restart can sometimes resolve temporary software glitches or conflicts that may be causing the backlight issue.

3. **Check for software updates**: Make sure your MacBook Pro is running the latest version of macOS. Software updates often include bug fixes and improvements that could address backlight issues.

4. **Reset NVRAM/PRAM**: Resetting the non-volatile random-access memory (NVRAM) or parameter random-access memory (PRAM) on your MacBook Pro can sometimes resolve display-related issues. To do this, restart your MacBook Pro and immediately hold down the Option, Command, P, and R keys simultaneously until you hear the startup sound for the second time.

5. **Check for physical damage**: Inspect the display for any physical damage, such as cracks or liquid damage, that could be affecting the backlight. If you notice any damage, you may need to have the display replaced by a qualified technician.

6. **External display**: Connect your MacBook Pro to an external display to see if the backlight issue persists. If the issue only occurs on the internal display and not the external display, it’s likely a hardware problem with the MacBook Pro’s display.

7. **Hardware diagnostics**: Run Apple’s built-in hardware diagnostics to check for any hardware issues with your MacBook Pro, including the display. To do this, restart your MacBook Pro and hold down the D key during startup.

8. **Contact Apple Support**: If the issue persists and your MacBook Pro is still under warranty or covered by AppleCare, it’s best to contact Apple Support or visit an Apple Authorized Service Provider for further assistance. They can help diagnose the problem and provide repair options if necessary.

Remember to back up your important data before attempting any troubleshooting steps or repairs, especially if you suspect a hardware issue that may require servicing.