No time to investigate at the moment. Some things we would suggest are trying with newer versions, disabling all collision before attaching, and try disabling bEnablePhysicsInteraction.
When possessing a vehicle, if you are standing on top of it with a character, once you begin to drive away, the vehicle will begin to be drawn back towards the character as if it is attached somehow. This results in the vehicle's physics going out of control.
This can be avoided by standing beside the vehicle when possessing or by destroying the character when possessing the vehicle but this is not a viable solution due to the limitations this causes.
Link to project: [Link Removed]
1. Download the project in the description, unzip it, and open it
2. Press Play
3. Walk up to the vehicle and jump on top of it.
4. Tilt the camera toward the vehicle and press E
5. After possessing the vehicle, begin to drive forward by holding W
Result: After a short time, the vehicle will be pulled back and begin doing barrel rolls back toward the character
Expected: The vehicle should drive off without seeming attached to the character
Workaround: Add a "Destroy Actor" node after the Possess node in the ThirdPersonCharacter Blueprint.
Head over to the existing Questions & Answers thread and let us know what's up.