How to reproduce it:
1. Open the attached project "1313365.zip"
2. Load the "SampleScene" Scene
3. Select a "Director" GameObject in the Hierarchy window
4. In the Timeline window, zoom to see Frames 0 and 1
5. Select a 'Sample Track' > 'Clip Properties' > 'Time'
6. Enter Play Mode
7. Observe the Time's property curve in the timeline

Expected result: Curve's keyframe is not created
Actual result: Playhead stops at frame 1, and the curve's keyframe is created. Keyframe modifies the Time's property curve

Reproducible with: 1.4.5 (2020.2.5f1), 1.6.0-pre.1 (2019.4.22f1, 2020.3.0f1, 2021.1.0b11, 2021.2.0a8)
Could not test with: 2018.4.32f1 (due to missing types and methods), 1.4.4 (2020.2.5f1)

Notes:
- Not always possible to reproduce from the first time
- If the keyframe doesn't appear after entering the Play Mode, repeat 6 and 7 steps
- In 2018.4.32f1, 'ClipEditor', 'CustomTimelineEditorAttribute', 'CustomTimelineEditor' types are missing and there is no suitable method found to override 'SamplePlayableAssetEditor.OnCreate(TimelineClip, TrackAsset, TimelineClip)' and 'SamplePlayableAssetEditor.OnClipChanged(TimelineClip)' methods