When a Sulfur Cube has a passenger (for instance, a mob or just a marker) its movement is not affected by other Sulfur Cubes. This means two ridden Sulfur Cubes can pass through each other instead of the expect behaviour, colliding.
Steps to reproduce
Run this command twice to summon two Sulfur Cubes with a Husk as a passenger:
/execute summon minecraft:sulfur_cube summon minecraft:husk run ride @s mount @n[type=minecraft:sulfur_cube]Feed both Sulfur Cubes with a block
Make them bounce around and they won’t collide with each other
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