I think it has something to do with the "battery saver" mode. Read the following.
Battery Saver
NOTE: The battery saver time-out is 1 minute if the vehicle has less than 80.45 km (50 mi).
NOTE: The battery saver does not control the parking lamps if the headlamp switch is in the PARKING LAMPS ON position.
To save battery voltage, the BCM provides automatic shut-off of the interior and exterior lamps after a time-out period when the ignition is off. The BCM monitors the ignition state and input from the RKE system to determine when to energize or de-energize the battery saver relay and to shut the power off to the lamps. A timer in the BCM starts when:
•the ignition changes to OFF,
•any door or liftgate becomes ajar while the ignition is off,
•an UNLOCK button of the RKE transmitter is pressed while the ignition is off,
•a valid keypad code is entered while the ignition is off,
•or the courtesy lamp switch (integrated into the FLM) is used to turn the courtesy lamps on while the ignition is off.
When 10 minutes have elapsed, the BCM automatically shuts off voltage to the lamps. The timer restarts (voltage is restored if the BCM is in battery saver mode) if:
•the ignition transitions out of OFF,
•any door or liftgate becomes ajar,
•the UNLOCK button of the RKE transmitter is pressed,
•a valid keypad code is entered,
•the courtesy lamp switch is pressed.
Courtesy Lamps
The BCM controls all the interior lighting functions and timing by monitoring inputs from the following:
•door ajar switches
•liftgate ajar switch (manual liftgate)
•LTM (power liftgate)
•ignition state
•courtesy lamp switch (integrated into the FLM)
•RKE system
•vehicle speed
The BCM sends a 12 volt signal to each door ajar switch and the liftgate latch (manual liftgate) or the LTM (power liftgate). When the doors and liftgate are closed, the voltage signals are pulled low, indicating a closed door or liftgate.
The BCM monitors input from the RKE transmitter (messaged from the TPM module on vehicles without IA or from the RFA on vehicles with IA) to determine when to activate/deactivate the illuminate entry feature .
The BCM monitors input from the FLM over the communication network to determine when to activate/deactivate the courtesy lamps. When the courtesy lamp button is pressed on the FLM, it sends a headlamp switch status message to the BCM indicating the request.
The BCM monitors the ignition state to determine when to activate/deactivate the illuminated exit feature.
Based on the input, the BCM controls the solid state relays to provide power to the courtesy lamps.
The BCM also provides an overload protection of the interior courtesy and puddle lamp output circuits. When an excessive current draw is detected, the BCM disables the affected interior courtesy or puddle lamp circuit driver. REFER to Section Field Effect Transistor (FET) Protection.
Illuminated Entry
The illuminated entry requests the dimmable backlighting, the parking lamps and the courtesy lamps to illuminate when the ignition is off and a door or liftgate is opened, the RKE transmitter UNLOCK button is pressed, or the vehicle is unlocked using the keyless entry keypad. If no doors are open, the illuminated entry requests the lamps off after 25 seconds have elapsed, the ignition is switched out of off, the RKE transmitter LOCK button is pressed, or the vehicle is locked using the keyless entry keypad.
