Reproduction steps:
1. Create a new Unity project with a 3D template (or just add a Timeline package to any project)
2. Create a Timeline in the Assets folder and add it to the Scene and save the Scene
3. Double-click on Timeline in the Assets folder and select Timeline in the Hierarchy window
4. Right-click on the left side of Timeline Editor and create any Track
5. Try clicking on the frame selector field (has 0 as default) and write in some number and notice how it works
6. Select the Timeline in the Hierarchy window and click on the frame selector field again
7. Close the project and reopen it
8. Repeat steps 6-7 once again
9. Select the Timeline in the Hierarchy window and try clicking on the frame selector field

Expected result: The frame selector field is clickable and looks editable
Actual result: The frame selector field is not clickable, but you can still write in a number and change it

Reproducible with: 1.4.4 (2019.4.20f1, 2020.2.4f1), 1.4.6 (2020.2.4f1), 1.5.2 (2021.1.0b6), 1.6.0-pre.1 (2021.2.0a4)
Not reproducible with: 1.2.18 (2019.4.20f1), 1.4.3 (2019.4.20f1, 2020.2.4f1)
Couldn't test with: 2018.4 as the Timeline package is not available