Fuel Gauge on Milwaukee M12 Brad Nailer Does Not Light When Powered On

According to the manual, page 5:
"To determine the amount of charge left in the battery, turn the tool on. The fuel gauge will light up for 2-3 seconds. When less than 10% of charge is left, 1 light on the gauge will flash 4 times."
However, the fuel gauge does light up whenever a nail is shot. I'm new to M12 and didn't realize this. Sorry.