Physics artifacts on terrain seams

Created two completely flat terrains, placed them adjacent to each-other. Then generated a composite collider (also tried with mesh collider and simple box collider) on a basic cube, added a frictionless physics material and applied a force to it so that it moves over the seam between the terrains. While the movement was completely smooth on top of each terrain, while crossing from one to the other it bounced, spined or otherwise reacted to some completely unexpected force. I am completely sure that the terrains are well placed, neither overlapping or with gaps between them. What is the cause of this? How can I address the problem?

This is a short video of the issue. The edge of each terrain is hastily painted gray for debugging.