When the colliders are joined, each cell that touches them only gets velocity contribution once. When there are multiple objects, velocities are summed, which causes some cells to get 2x, 3x or more than the actual object velocity. Fix this by using the average velocity of all colliders touching a cell. Reviewers: miikah, lukastoenne Reviewed By: lukastoenne Subscribers: dafassi, scorpion81, #physics Maniphest Tasks: T43782 Differential Revision: https://developer.blender.org/D2112