What exactly is this doing
Are you attempting to use field names that are not actually the field names?
ACF no longer allows getting fields using field names that are not actually defined in the fields or for fields that are defined somewhere else.
For example, trying to use
get_field('_thumbnail_id') for the post thumbnail instead of
get_post_meta($post_id, '_thumbnail_id', true) will result in an error/warning because this is not a field defined ACF (usually)
So of you are doing something that uses a field name other than the defined field name then you’re going to get this error. ACF is not finding a field defined with that name. The reason that using the field key does not create the error is that the field key never changes and ACF finds the field associated with the field key.
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