How to reproduce:
1. Open the attached "1279824 repro" project
2. Open the "SamepleScene" Scene from Assets > Scenes folder
3. Select the "TestObject" GameObject from the Hierarchy window
4. Open the Timeline window from Window > Sequencing > Timeline
5. Expand the Animation Track in the Timeline window
6. Select content of the track called "Override 0" in the Timeline window
7. Make sure that the Inspector window is in Normal mode

Expected results: the "Foot IK" checkbox is present in the Inspector window
Actual results: there is no "Foot IK" checkbox in the Inspector window

Reproducible with: 2018.4.29f1, 1.2.6 (2019.4.13f1, 2021.1.0a2), 1.5.0-preview.5 (2019.4.13f1, 2020.1.10f1, 2020.2.0b8, 2021.1.0a2)

Notes:
-In order to set "Foot IK" to true or false you need to convert the track to Clip Track -> Check/Uncheck the "Foot IK" checkbox -> convert the Track back to an Infinite Clip
-Another way to set "Foot IK" is to switch the Inspector window to Debug mode