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
});