From what I understand, you are trying to update an ACF field on a new post. From my experience, it is preferable to use the field key instead of the field name in the ACF functions – using the field name gets you in all sorts of strange situations – like the one you are having.
<?php update_field($selector, $value, $post_id); ?>
$selector (string) the field name or key (required)
$value (mixed) the value to save (required)
$post_id (mixed) Specific post ID where your value was entered.
Defaults to current post ID (not required).
This can also be options / taxonomies / users / etc
https://www.advancedcustomfields.com/resources/update_field/
In order to get the field key for your ACF fields, go to Custom Fields – your fieldset – Screen Options (top right) – Show Field Key = Yes
See this short tutorial about getting the field keys in ACF: http://thestizmedia.com/show-field-keys-acf-pro/