When wrong permissions are set to acf-json directory then no error is shown when saving the field group. After the group is saved all the fields looks like updated, so I assume that they’re saved in the database when acf-json dir is not writeable. That’s OK, but there should be at least a warning saying that acf-json directory exists but it’s not writeable.
I know it works like it was meant to work, but IMO it’s not a proper way.
I know that ACF JSON directory should be writeable and ACF documentation says about it.
I found this “bug” because the server configuration was wrong. It was OK on my local environment, but didn’t work on production. So it was a bit confusing and it wasn’t so obvious that ACF JSON directory is not writeable. I spent some time to figure it out.
IMO there should be a notice displayed, at least, when WP_DEBUG is set to true.