In some cases, the Auto Bezier spatial interpolation for Position keyframes can cause undesired back-and-forth (boomerang) motion between two keyframes with equal values. In such a case, you can change the earlier keyframe to use Hold interpolation or change both keyframes to use Linear interpolation.
To change the default to linear interpolation, choose Edit > Preferences > General (Windows) or After Effects > Preferences > General (Mac OS), and select Default Spatial Interpolation To Linear. Changing the preference setting does not affect keyframes that already exist or new keyframes on properties for which keyframes already exist.
For more information, including links to some video tutorials that demonstrate this problem and its solutions, see the "Controlling change with interploation" section of After Effects Help.
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