How to reproduce:
1. Open the attached 2018.3.14f1.zip project
2. Go to the Project window and select 724184.fbx
3. In the Inspector window-> Animation inspect the Length of the animation
4. Open the attached FBXTest.zip folder it has four folders with four animations of different lengths
5. Copy one animation with different length from the provided in the Unity project and then paste and replace that animation in the Project folder->Asstets
6. Go back to the Editor-> Project folder and select Scenes folder and select 724184.fbx
7. inspect the Length of the animation in the Inspector window
8. Repeat 2-7 steps to reproduce the issue (see the attached Repro-1202394.mov)

Expected: The total frames of the animation does not fill the timeline
Actual: The timeline is one second long, and the animation length is a subset of that

Reproducible with: 2018.1.0b6, 2018.1.b7, 2018.2.21f1, 2018.3.14f1, 2018.4.14f1, 2019.2.16f1, 2019.3.0f3, 2020.1.0a16

Not reproducible with: 2017.4.35f1, 2018.1.0b5

Notes:
- It is hard to reproduce, on 2018.1.0b5 I tired to reproduce for about 20min. Looks like it regressed in 2018.1.0b6