That is correct only opening the hatch turns on the boot light. If the CCCU thinks the hatch is open it will turn the lamp off after about 5 minutes. What happens if you have the hatch open, disconnect the battery and leave disconnected for 10 minutes then reconnect it? Remember if the car has a Gen ! radio you will need the code for it. Also the one touch windows, clock, date and very probably keys resync will also be needed.
My guess is the rear lock has an issue and needs replacing.