The easiest solution to the featured image issue is to change the field name of the ACF field to _thumbnail_id. However, you’ll still need to use the featured image functions in ACF for these fields until you manually update the post. It’s not a great solution.
The reason it will not work is that the acf field key reference will be missing. Every field in the postmeta table for an acf field has a matching field key reference.
post_id = post ID of the post the field is for
meta_name = "_($meta_name)" = the field name with an _ at the beginning. Following my above advice would mean "__thumbnail_id"
meta_value = The field key of the acf field, example: field_1235676
There is no easy way to do the second part. ACF will store a term ID. WP stores them as terms.
