It is likely you have two keyframes adjacent to each other at the end of the clip. This may be hard to see. Remove all the keyframes, and try again, this time making sure you only have one keyframe at the start and one at the end.
Note: once you put a keyframe in, if you move the playhead and change the setting, Premiere will automatically create a new keyframe at that point. Sometimes this behavior can generate unintended keyframes.
MtD