Relationship Fields
If you’re using “Relationship” field types in ACF and wonder why these fields are empty on freshly crossposted posts, it is because the same posts have different IDs on different websites.
All you need to do is to download and activate ACF Relationship Support add-on.
Download Simple Multisite Crossposting – Relationship Fields 2.1
Download Simple WP Crossposting – Relationship Fields 1.2
Remember, the posts listed in your “Relationship” fields, should be crossposted first.