Hi @OscarEriksen
I can confirm that the update_field function is working correctly with the repeater field!
I just added a repeater field to a page, edited the page.php template and ran this code:
$repeater = get_field('repeater');
// append row
$repeater[] = $repeater[0];
update_field('repeater', $repeater);
Looking back at the wp-admin edit page, I can see an extra row in the data!
Could this issue be that you are running your code before the repeater field is being included (before init)?
Thanks
E