I have a single WP site installation with a single main theme and ACF and ACF-Extended installed. Within the theme folder, I created the “acf-json” directory as stated in the documentation. To test the JSON file generation within acf-json directory, I went into a custom field group and saved it. Nothing happened. I disabled the ACF-Extended plugin, then repeated the steps, and the JSON files were generated inside the acf-json directory. Just to make sure, I enabled the plugin, and repeated the steps with another field group, and the JSON files did not get generated.
There is some conflict with the ACF-Extended plugin and the functionality of the acf-json generation.