Tim Dowling PRO said over 3 years ago on StimulusJS, Active Storage and DropzoneJS :
Thanks for pointing me in the right direction, it was working with the direct upload, but I was passing the blob signed id to the controller as a string rather than an array, this worked below:

 let formData = new FormData()
            formData.append("event[evidences][]", [this.hiddenInput.value] )
            Rails.ajax({
              url: this.data.get("update-url"),
              type: "PATCH",
              data: formData
              
            });