Steps to reproduce:

1. Create new dashboard with two widgets of maximum size using similar request:

   {
       "jsonrpc": "2.0",
       "method": "dashboard.create",
       "params": {
           "name": "ZBX-15596",
           "widgets": [
               {
                   "type": "discovery",
                   "name": "12x32",
                   "x": 0,
                   "y": 0,
                   "width": 12,
                   "height": 32
               },
               {
                   "type": "discovery",
                   "name": "12x32",
                   "x": 0,
                   "y": 32,
                   "width": 12,
                   "height": 32
               }
           ]
       },
       "auth": "c70164b94d5b7a23caa9054d431f9e53",
       "id": 1
   }
   

2. Open newly created dashboard in a browser
3. Click Edit dashboard
4. Click Add widget
5. Specify anything you want
6. Click Add

The last 3 steps can be repeated as many times as necessary.

Observed:
New widget is added at the end.