GitHub: https://github.com/Unity-Technologies/InputSystem/issues/594

Repro:
1. Download package at: https://github.com/Unity-Technologies/InputSystem
2. Download attached project: InputInBackground.zip
3. Make sure the "Run In Background" in Player setting is un-checked.
4. Build Player
5. Press Key 1, 2 or 3 to see what happens
6. Focus on other window or desktop, and press 1, 2 or 3
7. Re-focus on the Player window and observe

Actual: The Player window queues input when it is not in focus, and fires them all at once when it regains focus.

Note: I can repro this bug when using UI to construct Input Action, but cannot when only using scripts only.