Reproduction steps:
1. Create a new Unity project and open it
2. Import attached "1322543Test.fbx" asset to your project
3. Select this asset and look at the Inspector window
4. Select the Animation tab and uncheck the "Resample Curves" checkbox, set "Anim. Compression" to off, and check the "Animated Custom Properties" checkbox and click on Apply
5. In the top toolbar click on Window/Animation/Animation to open the Animation window
6. Expand imported asset and click on "Take 001" to view it in the Animation window
7. In the Animation window, select "Curves" to view animations in Curves mode and select "Animator.Eyelid Shape_L"
8. Look at frames 241-242

Expected result: No inconsistencies are observed, meaning that stepped keys were imported correctly
Actual result: A giant dip in the animation curve is observed between 241 and 242 frames

Reproducible with: 2018.4.35f1, 2019.4.27f1, 2020.3.9f1, 2021.1.0a7
Not reproducible with: 2021.1.0a8, 2021.1.8f1, 2021.2.0a17