Miles O'Brien, the Trekkie fellow from DS9?
He must get some stick with that name
It's strange how it has 'never' been seen before. Okay, the exhaust temperature is over 1000 Degs Celsius, and I think that the air would 'ignite' if there was a 'fuel' and oxygen present (most likely that they are).
However, to have at least a dozen night landing missions where it has never been seen and it's only just starting to happen, well, if I was a manager or a engineer associated to the APUs, I would be asking a lot of questions.
The only thing I can think of has the APU been modified to run a different mix of fuel recently?