It is not uncommon for the APU to have fuel leaks as some are considered natural leaks. These natural fuel leaks come from during and after the APU shut down process as the fuel nozzles may still be allowing a small amount of fuel to drip and collect in the combustor. Another leak considered natural is the fuel that collects or pools in the combustor on an APU failed start.
The combustor has a fuel drain line that connects to a spring loaded drain valve and allow this fuel to drain overboard. When the APU is not operating the drain valve is open and when the APU is operating this valve closes due to the internal pressure of the combustion chamber. The drain valve also prevents any air leaks from the combustion chamber when it is closed.
When the unburned fuel collects in the combustor, it is suppose to drain overboard but that is not always the case as it may also find its way to the exhaust cone. This can cause APU torching where an excess of unburned fuel can suddenly ignite at the next start causing a momentary flash or flames to come out of the exhaust accompanied with an audible bang. Just like the 747 and 757, the 787 is equipped with a fluid drip shield. This fluid drip shield helps drain or redirect any fluid away from going back into the exhaust, but if you go visit the 787 at the Flight of Dreams in Nagoya, Japan, the 787 at the Museum of Flight in Seattle, and the 787 at the Pima Air Museum in Tucson take a look at the APU exhaust. You won’t’ find this drip shield as it was added later in production to help prevent this problem.