I’ve come to a conclusion that the issue is a polylang issue rather than ACF issue. If one is using only “post__in” and “post_type” in wp_query, it shouldn’t make a difference in which language the request is made in as we has the IDs. Alternative solution would be to change ACF Core class-acf-field-relationship.php to iterate over every saved ID and map its content to a new array with “get_post”. “get_posts” with “post__in” does not work but “get_post” with single ID works.
Did some further debugging, apparently the relationship fields results use wp_query with “post__in” parameters with proper IDs, hence I feel like this should work out of the box but for some reason stays hidden.
Apparently if you change the return type to “id” you will get the hidden saved values out. Now the only thing missing is to figure out why saved values are not coming up as selected after a load.
I’m actually facing the exact same issue! Also weird is that when loading the field, no “non-translatable” CPT posts are visible, but when doing AJAX request, they become visible. After saving, nothing seems to be selected but in post_meta I can see that a non-translatable post is selected.
