Reproduction steps:
1. Download and open the attached "GamepadBugTest.zip"
2. In the menu bar, press "Window" > "Analysis" > "Input Debugger"
3. Connect a controller
4. Connect a second controller
5. Disconnect and re-connect the first controller
6. Press any button on the second controller

Expected result: The second controller's input is registered
Actual result: The second controller's input is not registered

Reproducible with: 2019.4.21f1, 2020.2.6f1, 2021.1.0b8, 2021.2.0a6 (Input System 0.1.2.preview - 1.1.0.preview.3)
Could not test with: 2018.4 (No Input System)

Notes:
- Tested with Xbox One wireless and Nintendo Switch Pro controllers.
- If both gamepads are connected before the editor is opened, then the issue does not reproduce. The editor first needs to be opened.
- Disconnecting and reconnecting the 2nd controller to be connected does not reproduce the issue.