How to reproduce:
1. Open the attached "1272521 repro" project
2. Open the "A" Scene from Assets > Scenes
3. Enter Play Mode
4. Press the "Space" key on the keyboard
5. Click the Button in the middle of the Game window

Expected results: the Button is clicked successfully
Actual results: the Button is unclickable

Reproducible with: 0.2.10-preview (2020.2.0b1), 0.9.0-preview (2020.1.4f1), 1.0.0 (2019.4.9f1, 2020.2.0b1), 1.1.0-preview.1 (2019.4.9f1, 2020.2.0b1)
Could not test with: 0.1.2-preview (2019.4.9f1, compilation errors)

Notes:
-Turning on the Debug mode on the Inspector window and selecting the "EvenSystem" GameObject in the "B" Scene after the 4th step will reveal that the "Owns Enabled State" variable is set to false
-The same behavior can be observed in the "C" Scene where disabling/deleting the "EventSystem (main)" GameObject leaves the button unresponsive since the other Input System UI Input Module, present in the Scene, doesn't become active