This is a quick update, not sure if it should be considered a bug or not but CustomPhysicalProperties aren’t applied unless done via a script, if it’s done when the game isn’t running it won’t work as intended, but if you make a script do it while the game is running it will.
Read this carefully. The behavior of the parts are determined by the average of their physical properties with their colliding surface. It is most likely because the ground the parts are landing on is not customized.
Although the formula above refers to the Friction and FrictionWeight of two parts, A and B, the formula is used in the same manner when determining Elasticity. Generally, when the weight of A is much greater than that of B, the actual value will be closer to A. If the weights are similar, then the actual value will be close to the midpoint between their individual values.