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/
