[Unity 2018.2.5f] Grenade bounce deviates after collided the point which divides 2 flat prefabs

I’m making a level with the modular building, but a bugged bounce happens when the grenade touches the separation between modules.
Did somebody here meet the same problem in past? I’ve got a friend that is making his game and he is more expert than me with Unity, but despite his talent and knowledge (I’ve sent him the scene to check if the colliders or something else is messed up), we can’t state the solution.

I could model the levels in Blender, but the modular building allows you to work easily and above all modifying a level very fast without modeling again a place if it doesn’t fit well with game mechanics.

We tried to:

-Set grenades to Continuous dynamic

-Substitute Mesh collider with Box collider

-Checked the size of the modular pieces and their colliders.

-Modify the Default collision offset (aka Min Penalty)

But nothing was fixed.

Moreover, it happens to the player character too. When it is stopped between two modular pieces (green and white for instance) it happens a little and slow movement as if the Y position of either of these two is “higher” than the other, making the player translate towards the “lower” one… but both have the same Y position, rotation, scale, and box collider size.

Here’s a video that shows the bugged bounce.

Some help?

Sorry, you probably tried this already but just in case are you sure don’t have another gameobject that has a collider? Maybe it’s even a child of the prefab and that would make it happen on every module