I'm not sure if this is a bug or not, but It's not the desired behavior.
The small falling boxes on the corners should stack up, but instead they "move" the box on the base.
Is there any way to fix this?
You should not use the Platform behavior for stacking falling objects, it's designed for jump-and-run player control. Maybe give the Physics behavior a shot?
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Ok, I suppose that a physics behavior with the correct parameters could give the same effect :)
But, I still think it's a little weird because the other boxes stack perfectly.
I had the exact same issue and didn't want to use physics objects for blocks because I simply wanted blocks that you could push and stack. I fixed this issue by making sure that when a block is not being interacted with and is on the floor, it Disables its platform behavior. This fixed it and got me the pushable block mechanics I wanted.
Just a side-effect of using something for something outside of its intended purpose ;)
I'd look at Physics, as Ashley said, or Custom movement.