Editor Window Modes (Mix Mode, Ripple Mode and Replace Mode) cannot be switched using hotkeys.

Steps to reproduce:

1) Open a new unity project
2) Open Timeline window by clicking on Window > Sequencing > Timeline
3) Create a gameobject and then select it
4) In timeline window, click on Create button
5) Save new playable asset
5) Use hotkeys 1, 2 and 3 to toggle between Mix Mode, Ripple Mode and Replace Mode
Observe that the mode is highlighted /switches for a brief amount of time and goes back to the previously selected mode.

Actual result: Switching modes using hotkeys does not work

Expected result: Switching between modes using hotkeys should work like it would by using mouse clicks

Reproduced with:
2019.1.0a8, 2018.3.0b10, 2018.2.7f1, 2018.1.9f2, 2018.1.0a6

Environment: Occurs in Windows 10, Mac OS 10.13

Note: The buttons for Mix Mode, Ripple Mode and Replace Mode were introduced in 2018.1.0a6