Multible checkbox selection in ACF does not work

my values

s1 : red
s2 : blue
s3 : green

i want to select s1 and s2

update_field( "eigenschaften_sicherheit", array("s2", "s1"), 5268 );

this code set only the latest s2 value and not the s1

for exampe this code will only select s3

update_field( "eigenschaften_sicherheit", array("s1", "s3"), 5268 );

The docs say this

// save a checkbox or select value
$field_key = "field_1234567";
$value = array("red", "blue", "yellow");
update_field( $field_key, $value, $post_id );

any ideas whats wrong with my code?