I was adding the hook inside the custom field class __construct:
class acf_field_my_custom_field extends acf_field {
//ajax hooks here
}
That’s in the acf-my_custom_field-v5 file.
I couldn’t get it to work there, so I added out side the class in the main plugin file, and it worked. However that creates another problem… I don’t have access to the data inside the class my_custom_field.