I have been working for an airline operating A340, A330 for more than 15 years never come across such problem, it could be due to the aircraft configuration as all this fleet is 3 classes. The forward toilet always used by first class pax and flight deck crew and it is always tidy & clean. I would thought this is depends on who is using the loo and how they look after it. As the guys said it could be due to system leak and faulty extract fan.