Actually this caused a regression :
fields that take a "list" value (as a php numeric array), now receive a JSON object with explicit numeric keys :

example :

$client->coupon()->createForItems([
    'id' => 'foo',
    'name' => 'foo',
    'discount_percentage' => 10.0,
    'apply_on' => 'each_specified_item',
    'item_constraints' => [[
        'constraint' => 'specific',
        'item_type' => 'plan',
        'item_price_ids' => ['some_price_id'],
    ]],
]);

--> Chargebee\Exceptions\InvalidRequestException  item_constraints[item_price_ids][0] : wrong format.

in the final JSON payload,
item_price_ids is now sent as {"0":"some_price_id"} (instead of ["some_price_id"])
and this format is rejected by the API server

--> maybe the JSON_FORCE_OBJECT param should be used only if the $params array to serialize is an empty array ?