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The scene capture will revert back to the Character BP that is placed in the level when player 2 is not moving. When they are moving the scene captuer will show correctly.
You can get around this by deleting the Character BP from the level and spawning in using Player Starts. Placing the Character BP back in the level will cause the issue again.
Attached Sample Project that recreates this and video for demonstration.
1. Create a new "Third Person" project.
2. Create a new Render Target texture and a new material for Scene Capture.
3. Add a SceneCaptureComponent2D to ThirdPersonCharacter blueprint.
4. Create a widget blueprint and place a Image widget on the left corner or somewhere.
5. Assign the material we created for Scene Capture to the above Image widget.
6. The material will ask to be switched to the UI domain. Click the red text under the image.
7. double-click on the material to open it up and reconnect the material
8. Open Level Blueprint
9. BeginPlay Event -> Create Widget -> Add to Viewport
10. Change "Number of Players" to 2 in Play options
11. Click "Play" and watch the 1st player's captured image when 2nd player moves
Results: When a Character BP is dropped in the world the scene render will default back to this instance when the other player is not moving.
Expected: Player 2 should only have it's own scene render capturing what it sees.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-21923 in the post.