First problem is that the format you’re attempting to store is incorrect. ACF may display and return values in the format you set but it always stores date fields in the format “YYYYMMDD”
The second problem could be related to the adding a new post where the custom field does not exist yet or updating and existing post that already has a date value. When using update_field() for new post you always need to use the field key and not the field name.
