I am seeing the same issue.
Just created a new theme and am finishing it off by integrating ACF Pro in theme directory.
This worked fine of course.
Only issue is, that the acf-json directory in my theme’s directory remains empty, even though I tried updating the existing field groups several times.
What am I doing wrong? this is not a child theme.
Best
Saskia
EDIT: found the issue.
I added the directory using my ssh user. Thus the dir was owned by my ssh user and even though permissions were set to 755, the WordPress installation was unable to write to it, because WP makes use of www-data user (id 33 in my ubuntu install) that is in www-data group (id 33 in my ubuntu install). To solve this issue you must run following command on your vhost directory (replace with user ids or username and group of your server setup:
chown 33:33 -v -R /var/www/path/to/your/domains/rootdir/
