Actually var_dump did a job for me within my function, and in my case revealed that plugin_dir_url( __FILE__ ) was providing a url ending with / so my constructed path of:
plugin_dir_url( __FILE__ ) . ‘/acf-json’
(which similar to yours above skrabbel features a / in the final part) was producing an erroneous // in the URL.
….frustratingly though this hasn’t resolved my issue (which has similar symptoms to the above – filtering load_json within a plugin failing to then implement the field (group) ), and the above prioritizing doesn’t either.