Kinematic "buckets" no longer working in 2018

My VR game involves a sushi bar with a conveyor belt system that carry a bunch of rigidbody, interactable pieces of sushi. The conveyor belt is a bunch of boats that are kinematic rigidbodies movies around with Rigidbody.MovePosition at a slow, constant speed. The box colliders that form the boat "bucket" have a material on them with a slightly higher coefficient of friction to ensure the little pieces of sushi don't slip off. I also set both the boat and the sushi to continuous dynamic and interpolated interactions.
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Back in 2017.1, this worked just fine. The sushi, whose rigidbodies consist of a simple box collider, sat in their boats as the moved along a path.
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When I upgrade to 2018.1.4, the pieces of sushi suddenly would no longer track along with the boat rigid bodies. They sort of freeze up, get stuck, and then fall off the back. It's almost like the physics engine really doesn't want to deal with those interactions anymore, and it's super bizarre.
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I made some changes so that the boats are now no longer Kinematic and are using velocities rather than MovePosition, and that works just fine, though I had to jump through a bunch of hoops to make sure they only interact with the sushi, and can't be knocked off their path by other physics interactions.
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Does anyone have an idea why in 2018.1.4 a non-kinematic rigidbody would suddenly have so much trouble "riding" on a kinematic rigidbody?