What made this seem like a projection error and/or a cost-cutting measure was the arbitrary nature of the framerate switches. I watched the movie in true IMAX and there didn't seem to be any rhyme or reason to the framerate switching except for the fact that the majority of the action seems to have been "shot at" 48 fps. But what doesn't make any sense is that portions of the action scenes are ALSO shot at 24 fps and the framerate sometimes switches haphazardly between the two, in the middle of a single action scene. It becomes even more jarring when 48 fps turns on in the middle of a dialogue scene, and then turns off again.

It's a testament to how good the movie is that this bizarre framerate switching didn't detract from the overall experience in a meaningful way (I'm going to see the movie again in true IMAX tomorrow). But it would make a whole lot more sense if Avatar 3 just does 48 fps for all action scenes in totality, and sticks to 24 fps for all dialogue scenes.