The engine still uses the default index for calling force feedback events, despite them being assigned from player 1 on out.
Which is why event on player 1 goes to player 0, event on player 2 goes to player 1.
a. Attach 1 or 2 Xbox Controllers
1. Download attached project
2. Note that: In Project Settings -> Maps & Modes -> Local Multiplayer -> Enable "Skip Assigning Gamepad to Player 1" is enabled
3. Open "ThisPlayerController"
4. Call rumble events on player controller indexes and observe offset
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-56077 in the post.