I’m using the latest fbx plugin 2013.3 and Maya 2013 64 bits. I have found a king of bug while importing fbx file with option “Update animation only”.
The bug is because my pelvis bone channel Y trajectory is constant.
Importing animation into Maya I get the Y channel without any keyframes, It seems that Constant Key Reducer works by default within the plugin. What it promises unpleasant? When I want to load another animation over the existing, empty track will not override anything, because it is empty, and the trajectory of moving Y remains unchanged. Thus, we get a new movement with the old and new trajectories. That is, to make sure that the animation is fully from fbx clip, remove all animation keys from objects that are expected to import new animation.