bulkUpdate doesn't seem to work with multiple-reference field.

With my following code the 'title' property is correctly updated but the multi-reference field 'tag' no.

Why this?

export function update() {

     let id1 = 'b4d0f567-d370-46bb-9a7f-9b383065bd3e' //id of an item in 'tag' collection
     let id2 = '635c6630-5c96-454f-8b38-32336fe2b765' //id of an item in 'tag' collection

     let toUpdate = [
        {
            _id: '7d9cbc8e-86bb-45d7-9485-9fa3805a84e1',
            title: 'testBulk 2',
            tag: [id1,id2]
        }
     ]

     wixData.bulkUpdate("productData", toUpdate)
            .then( (results) => {
                console.log('update results: ', results)
            } )
            .catch( (err) => {
                console.log(err)
            } );

}