How to reproduce:
1. Open the attached "SpotlightInnerAngle" project
2. Open the "SampleScene" Scene
3. Open the Animation window (Window->Animation->Animation)
4. In the Hierarchy select the "Spot Light" GameObject
5. Start recording in the Animation window
6. In the Animation window's Timeline section go to the 0:30 time mark
7. Change the "Spot Light" GameObject's Inner Spot Angle to 0

Expected results: In the Animation window's Timeline section a keyframe is added
Actual results: In the Animation window's Timeline section no keyframe is added

Reproducible with: 2019.3.0a12, 2019.4.8f1, 2020.1.3f1, 2020.2.0a21 (7.0.1, 10.0.0-preview.26)
Could not test with: 2018.4.26f1, 2019.3.0a11 because the Spot Light's Inner Spot Angle was added in Universal RP 7.0.1 which is available in 2019.3.0a12

Notes:
- Spot Light's Inner Spot Angle option only exists in Universal RP and High Definition RP
- The issue is present only in Universal RP, in High Definition RP you can animate the Inner Spot Angle (The property can be added by going to HD Additional Light Data->Inner Spot Percent)