I own two Sound Devices 702t recorders. Incredibly tough machines. Unfortunately, not user repair friendly unless you are an experienced technician. It’s likely the LED backlights for the LCD panel on that side have either failed or developed a bad connection. The LCD screen itself shouldn’t require replacement as long as all of the characters are evenly visible with the backlight off. If you don’t want to risk further unplanned malfunctions, I’d send it to a SD aware tech, or best yet send it back to the mothership for a going over (they will repair any issues and clean it thoroughly).