I had a similar problem, but it was not only on upgraded fields ; but also on new created fields ; nothing saved.
I found that it was the post_name which cause problem.
— post_excerpt is a readable slug
— post_name is a string starting with “field-”
If I change post_name to start with “field_” ; I was able to save my data.
I just run this SQL query and everything seems to work :
UPDATE wp_posts SET post_name = CONCAT( 'field_', SUBSTR( post_name, 7 ) ) WHERE post_type = "acf-field" && post_name LIKE "field-%"
