The game crashes when two pistons, two blocks apart, facing each other, push (at the same time) a custom entity in the space between the two pistons. This is the entity file:

 
{{{}}
    "format_version": "1.16.0",
    "minecraft:entity": {
        "description": {
            "identifier": "namespace:example",
            "is_summonable": true,
            "is_spawnable": true,
            "is_experimental": false
        },
        "components": {
            "minecraft:collision_box": {
                "height": 1,
                "width": 1
            },
            "minecraft:physics": {
                "has_collision": true,
                "has_gravity": true,
                "push_towards_closest_space": true
            },
            "minecraft:pushable": {
                "is_pushable": true,
                "is_pushable_by_piston": true
            },
            "minecraft:health": {
                "value": 1,
                "max": 1
            },
            "minecraft:scale": {
                "value": 1
{{            }}}
{{        }}}
{{    }}}
}
 
I attached a recording of the crash, in action. As you can see, the game crashes when the pistons are activated at the same time.
NOTE: vanilla entities with and without AI do not seem to be affected by this bug (i tested with pigs and armor stands).