Sometimes I've noticed that the distal half of one of the furthest posterior teeth is missing from an aligner. Why is that?
Sometimes we receive impressions that don't have enough detail to accurately model the distal edges of the posterior teeth. Distortion is much more prevalent in this area, because it can be difficult to make sure that the impression material completely covers the teeth in the back of the mouth.
Of course, we always prefer to receive complete, accurate impressions. But we don't want to inconvenience our providers unnecessarily either.
In some cases, we'll make an exception and process the case even though the distal surfaces of the posterior teeth are incomplete in our model. The aligners still have plenty of surface area to grip the teeth. We just trim off the potentially inaccurate area so that the case can progress without delay.
If you want to make sure that your patient's aligners fully cover the distal surfaces of all the teeth, just double-check your impressions to make sure that they're not distorted in that area, and you should be fine.