I would need to have more information on what you’re trying to add this to.
The only thing that I can think of here is that whatever you’re trying to add the ACF fields to is not processing the submission in a way that allows ACF to work. And if that’s the case then your manually trying to do so may not work either.
Also, if you are trying to update fields manually where data is not already saved then you need to use the ACF function to update the fields and you need to use the field key.
$acf_start_date = $_POST['acf']['field_60168af55da39'];
update_field('field_60168af55da39', $acf_start_date, $post_id);
But all these are just guesses.
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