DIAGNOSIS The drill is fine. This is a battery pack issue. One or more cells inside the pack are failing, causing the battery's internal management system to report low voltage under load.
FIX Test with a different, known-good DeWalt 20V Max battery. If the drill works normally and shows a correct charge with the other battery, you've confirmed the first battery pack is faulty. Replace the battery.
IF THAT DIDN'T WORK If multiple batteries show the same one-bar symptom in this drill but work fine in another DeWalt tool, then the problem is in the drill's battery terminal contacts or the circuit board that reads the battery data. Clean the contacts first with isopropyl alcohol.
HEADS UP Do not continue using a battery that shows one bar when you know it's charged. It can fail catastrophically (overheat, swell). A swelling battery is a fire hazard—dispose of it properly immediately.