It is possible, that physycs engine will not properly set collisions between rigid and kinematic bodies. What propably happens is kinematic platform moving by overriding its global_position and Rigid bodies falling with gravity force into kinematics collision shape. I would suggest to make platform a RigidBody also, but with Kinematic mode ( There are 3 modes of rigid bodies - rigid, static and kinematic ). If You can’t do this You may eventually tweak mass/gravity of rigidbodies or make kinematic collision shape thicker, with big margins.
Thanks a lot for the answer. I think that the problem is related with the stacking-bodies bug in godot, and I was able to solve it by increasing the physics fps to 180.