When you update the repeater field the values need to be in a nested array, you will also need to use field keys instead of field names.
// replace field keys with your real field keys // Sponsor -> field name(repeater field) - field key (field_XXXXXXX') // Select sponsors -> Sub field.(relationship) - field key (field_YYYYYYY') $repeater_value = array( // each nested array is a row in the repeater array( // each row contains field key => value pairs for each sub field // select field 'field_YYYYYYY' => array(1142,1141) ) ) // update field using repeater field key update_field('field_XXXXXXX', $repeater_value, $post_id);
alternately you can use update sub field explained here https://www.advancedcustomfields.com/resources/update_sub_field/
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