To reproduce:
1. Open the attached project "New Unity Project (3).zip"
2. Select Handgun_fbx_7.4_binary
3. In the inspector window select Animation tab
4. Select any of the animation clips with 1 frame
5. Observe the animation timeline bellow

Expected: the first frame 0 is aligned to the left
Actual: the first frame is pushed to the right

Reproduces on:
2018.4.26f1 2019.4.4f1 2019.4.6f1 2020.1.0b16 2020.2.0a16

Doesn't reproduce on:
2018.4.25f1 2019.4.3f1 2020.1.0b15 2020.2.0a15

Notes:
- Tested on macOS 10.14.5
- All versions that were marked as not reproducible, getting this issue instead: https://fogbugz.unity3d.com/f/cases/1202394/