How to reproduce:
1. Open the attached "1227651.zip" Unity project
2. Open the Timeline window (Window > Sequencing > Timeline)
3. Select the "TimelineTimeline.playable" in the Project window
4. Expand the "Animation Track (4)" and find the end keyframes
5. Move the orange keyframe side to side, stretching the white box

Expected result: the selected keyframe can be moved without moving other keyframes
Actual result: the selected keyframe stretches the white box and moves other keyframes from other curves

Reproducible with: 2019.3.16f1, 2019.4.3f1, 2020.1.0b14, 2020.2.0a17 (Timeline 1.2.12, 1.3.0)
Not reproducible with: 2018.4.24f1, 2019.3.4f1 (Timeline 1.2.11), 2020.2.0a17 (Timeline 1.3.1, 1.4.0)

Notes:
1. Reproducible on Timeline package versions 1.2.11 through 1.3.0