Enscape v3.0.0
Revit 2021.1.2
Adjusting the timestamp on keyframes toward the end of the timeline will force adjust the Total Duration instead of creating more or less space between the last keyframe and the previous keyframe. I expect the total duration to be fixed, per the Timeline Total Duration setting, and interior keyframes to move relative to the overall time.
See attachments for an example of adjusting the timestamp on the second to final keyframe, and the final keyframe (total duration) adjusting on its own.