I’ve made some progress – ACF fields must be added to the post following the wp_insert_post() call if they are to be used in the theme!
In V4, it seems that calling update_field would add the data to the post and calling get_field() ultimately returned something regardless of whether the post had been updated or not.
In V5, calling get_field() on uninitialized fields will return NULL until the post is updated through the WP admin!
My theme was doing logic checks to see if a field was true or false before printing some text. Before upgrading, it didn’t matter if the ACF field was actually populated with data, but now it seems that the field will not return a boolean value (instead returns null) if update_field() was never called.
Simple solution – call update_field() on all ACF fields directly following the wp_insert_post() call!
