@Lilly, diabetesmine actually raised that question a bit ago, here is what they said:
"Leftover Powder – I was also very worried about seeing some residual white powder left in the cartridge after most doses: was all the insulin really getting into me?
But Boss [Sanofi Senior Medical Director] reassures us that a little leftover powder in a used cartridge should not cause concern. The manufacturers [of] Afrezza cartridge[s] don’t expect 100% absorption, which is also the case with injected insulin, he says. “It’s OK if about 8% still seems to be in the cartridge afterwards. That’s calculated in. You lose some in the mouth, some in the cartridge. It’s such a fine powder that most of it absorbs in the lungs.”
(from https://www.healthline.com/diabetesmine/six-things-know-starting-afrezza#2)
If you read this thread we had in October, users say they sometimes cough some up but don’t see a difference in behavior afterwards: