Unfortunately solely Tweening the .Position doesn’t include the physics engine, which is what would cause characters to move along with the platform. I would recommend using a BodyMover, specifically BodyPosition, to set the platform in motion, since this works using physics.
Here is another couple posts already on this topic:
To solve this, you can incorporate a BodyGyro as well as a BodyPosition. This allows you to stabilize and lock the orientation via a CFrame input, which you could probably set to a empty CFrame depending on your platform’s desired orientation.