Don't double-encode submitted data.

app/Http/Controllers/ResultController.php

@@ -183,7 +183,7 @@ class ResultController extends Controller
      * API route for submitting results for the experiment.
      */
     public function submit(Request $request) {
-        $data = json_decode($request->data);
+        $data = $request->data;
         $run_id = $request->id;
         $isPartial = $request->partial ? true : false;