With the amount of cargo on MJ's plane, at times it's impossible for customs to check each and every detail.
The doctor would have a case or crates of medication. They would peruse through, look at the inventory and clear it.
You think 2 or even 10 customs staff have the time to open each and every crate and suitcase item in a fully loaded Boeing 747 jet?
You see checking just one bag thoroughly takes them ages.
MJ's doctor would have declared the drugs one way or another. Even if it's declared as propofol, firstly the customs guy wouldn't have a clue what propofol is, and as long as it's not listed on prohibited drugs, it would have cleared as part of a medical consignment.
At customs the doctor does not need to declare how they will use the medicine, so they can carry what may be extra medication that may not be used.
Other celebrities who travel with doctors definitely the doctor has packs of different drugs and with customs i believe all they do is to check amount carried is not too excessive and also not prohibited.
Think of those who have high blood pressure or other conditions, they have packs of medicine carried by doctors which are not purchased locally.
An anaesthetic I believe would easily be cleared