OK, here’s a better version. You have to find your ACF field, find the textarea within the editor, get that ID, then setContent using TinyMCE JS API:
var description = "Some text!";
var f = acf.getField("field_5d5231b7e8738");
var tinyID = f.$el.find("textarea").attr("id");
var tinyInstance = tinyMCE.editors[tinyID];
tinyInstance.setContent(description);
Worked well for me, let me know how it goes.
