Thumbstick clicks do not register on Linux

The "Left Thumbstick Button" and "Right Thumbstick Button" events on Linux do not trigger with the thumbstick is clicked. Testing all gamepad input, clicking the thumbstick in appears to call the Left/Right Thumbstick Down event.

This does not occur on Windows - Thumbstick Button events do trigger when thumbstick is clicked.

Regression:
No - Same behavior occurs on 4.15.3 on Linux (CL 3450819)

1. Connect XBox controller to Linux machine
2. Open Third Person Template project
3. Open ThirdPersonCharacter blueprint
4. Add Event Gamepad Left Thumbstick Button
5. Wire event node to PrintString node
6. PIE
7. Click the left thumbstick in

Result:
Print Message does not appear on screen

Expected:
Clicking the thumbstick button triggers the "Thumbstick Button" event