Looked into this a bit more now, and it appears that the ACF PRO action save_post in forms/post.php is never called in the current state of my website here.
Edit: Seems to be different, still testing…
Edit 2: Seems to be problem around suppress_filter.
With suppress_filter=false and WPML active, trying to get a field in _acf_get_field_by_key() does not return data.
If I change _acf_get_field_by_key() to suppress_filter=true, it looks like the correct post is returned by get_posts() but it actually is the opposite language, so a query with acf_field_key=field_1 returns a post with post_name=field_2, weird.
Example log with suppress_filter=true
acf/api/api-field.php/acf_get_field
$selector=field_53c599718b60c
$k=key
$v=field_53c599718b60c
returns a $field with
$field[ID]=4032 (wrong language)
$field[label]=Adresse (wrong language)
$field[key]=field_53c5986bcd16e
Note the not matching key.
