For about a year, I have been contacting support about an issue with my MX Keys S keyboard. The backlight would not come on when my hands came near the keyboard. Today I figured it out why. If the keyboard is plugged in to recharge the batteries, the proximity sensors do not work. When unplugged, they do. I usually keep the keyboard plugged in because the battery life is pathetic. Today I unplugged the keyboard and the backlighting started working when my hand was near the keyboard. I tested it several times to confirm that it only works if not plugged in. The backlighting will come on when plugged in if a key is pressed, but not when the hands get near the keyboard.