REST API returning "invalid" JSON on some environments

/query/service endpoint returns "invalid" JSON on production, but exactly same query returns correct JSON on my machine.

The "invalid" JSON looks like:

{

  "requestID": "...",

  "signature": "{\"foo\": \"json\"}",

  ....

}

when the valid response on my machine looks like this:

{

  "requestID": "...",

  "signature": {"foo": "json"},

  ....

}

Both computers are Windows and running Couchbase 6.0.0 build 1693.

This happens when I try to execute an encoded_plan trough the REST API, my request looks like:

{"args":["args","here"],"prepared":"id here","encoded_plan":"encoded plan here","encoding":"UTF-8","creds":[{"pass":"...","user":"..."}],"timeout":"75s","scan_consistency":"not_bounded"}