autocomplete_ajax returns an empty response

[rdlmda]

WP 4.8.1 (from official Docker image) ACF 5.6.1

{
  "log": {
    "version": "1.1",
    "creator": {
      "name": "Firefox",
      "version": "54.0"
    },
    "browser": {
      "name": "Firefox",
      "version": "54.0"
    },
    "pages": [
      {
        "startedDateTime": "2017-08-22T13:45:10.064-03:00",
        "id": "page_1",
        "title": "Adicionar fotografia ‹ SITE NAME — WordPress",
        "pageTimings": {
          "onContentLoad": -1,
          "onLoad": -1
        }
      }
    ],
    "entries": [
      {
        "pageref": "page_1",
        "startedDateTime": "2017-08-22T13:45:10.064-03:00",
        "time": null,
        "request": {
          "bodySize": 0,
          "method": "GET",
          "url": "http://localhost/wp-admin/admin-ajax.php?action=autocomplete_ajax&request=bra",
          "httpVersion": "HTTP/1.1",
          "headers": [
            {
              "name": "Host",
              "value": "localhost"
            },
            {
              "name": "User-Agent",
              "value": "Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0"
            },
            {
              "name": "Accept",
              "value": "application/json, text/javascript, */*; q=0.01"
            },
            {
              "name": "Accept-Language",
              "value": "en-US,en;q=0.5"
            },
            {
              "name": "Accept-Encoding",
              "value": "gzip, deflate"
            },
            {
              "name": "x-requested-with",
              "value": "XMLHttpRequest"
            },
            {
              "name": "Referer",
              "value": "http://localhost/wp-admin/post-new.php?post_type=fotografia"
            },
            {
              "name": "Cookie",
              "value": "REDACTED"
            },
            {
              "name": "Connection",
              "value": "keep-alive"
            }
          ],
          "cookies": [],
          "queryString": [
            {
              "name": "action",
              "value": "autocomplete_ajax"
            },
            {
              "name": "request",
              "value": "bra"
            }
          ],
          "postData": {
            "mimeType": "",
            "params": [],
            "text": ""
          },
          "headersSize": 1824
        },
        "response": {
          "status": 200,
          "statusText": "OK",
          "httpVersion": "HTTP/1.1",
          "headers": [
            {
              "name": "Date",
              "value": "Tue, 22 Aug 2017 16:45:10 GMT"
            },
            {
              "name": "Server",
              "value": "Apache/2.4.10 (Debian)"
            },
            {
              "name": "X-Powered-By",
              "value": "PHP/5.6.31"
            },
            {
              "name": "x-robots-tag",
              "value": "noindex"
            },
            {
              "name": "X-Content-Type-Options",
              "value": "nosniff"
            },
            {
              "name": "Expires",
              "value": "Wed, 11 Jan 1984 05:00:00 GMT"
            },
            {
              "name": "Cache-Control",
              "value": "no-cache, must-revalidate, max-age=0"
            },
            {
              "name": "X-Frame-Options",
              "value": "SAMEORIGIN"
            },
            {
              "name": "Content-Length",
              "value": "2"
            },
            {
              "name": "Keep-Alive",
              "value": "timeout=5, max=100"
            },
            {
              "name": "Connection",
              "value": "Keep-Alive"
            },
            {
              "name": "Content-Type",
              "value": "text/html; charset=UTF-8"
            }
          ],
          "cookies": [],
          "content": {
            "mimeType": "text/html; charset=UTF-8",
            "size": 2,
            "text": "[]"
          },
          "redirectURL": "",
          "headersSize": 407,
          "bodySize": 2
        },
        "cache": {},
        "timings": {
          "blocked": 0,
          "dns": 0,
          "connect": 0,
          "send": 0,
          "wait": 128,
          "receive": 0
        },
        "serverIPAddress": "127.1.0.1",
        "connection": "80"
      }
    ]
  }
}