$metaDataList = [];

foreach ($precontractData->items as $i => $item) {

    $metaDataList[] = [

        'key' => "item_description{$i}",

        'value' => $item->name,

    ];

    $metaDataList[] = [

        'key' => "item_quantity{$i}",

        'value' => intval($item->cuantity),

    ];

}

You are constructing your array wrongly. There is no keys named item_description_0, just key and value. So you need to adjust your loop:

$metaDataList = [];

foreach ($precontractData->items as $i => $item) {

    $metaDataList[] = [

        'key' => "item_description{$i}",

        'value' => $item->name,

    ];

    $metaDataList[] = [

        'key' => "item_quantity{$i}",

        'value' => intval($item->cuantity),

    ];

}

Final $metaDataList will be similar to

[

    [

        'key' => 'item_description0',

        'value' => 'Name',

    ],

    [

        'key' => 'item_quantity0',

        'value' => 100,

    ],

    [

        'key' => 'item_description1',

        'value' => 'Name 1',

    ],

    [

        'key' => 'item_quantity1',

        'value' => 101,

    ],

];