OK, it worked correctly up until one of the recent updates. May be different bug has to be filed?

Bisect info: 451173 (good) - 451195 (bad)
https://chromium.googlesource.com/chromium/src/+log/0d0d5e9e..00cd4a4b?pretty=fuller
Suspecting r451194 "Do not guess UTF8 encoding"
Related: issue 691985

Evidently, an intended change:
  UTF8 auto-detection can allow/encourage web publishers
  to neglect proper encoding labelling and rely on browser-side encoding
  detection. This CL helps prevent that.

However, Chrome's new behavior seems incorrect in this particular case because the default encoding for JSON is UTF-8 as per specification [1] so even though the server in question doesn't explicitly set the encoding header, it sets "Content-Type: application/json", which should have changed the default encoding to UTF-8.

  [1]: https://tools.ietf.org/html/rfc7159#section-8.1

As per Comment# 3 assigning to the concern owner from the provided CL.
Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/89967f02f2402870acb8322ad50c391b3a0971e7

@jinsukkim -- Could you please look into the issue, kindly re-assign if this is not related to your changes.
Thanks in Advance.

Thanks for the report. Will work on the fix.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0a6f6a195d44070c256172a2ca0c8353d92b596d

commit 0a6f6a195d44070c256172a2ca0c8353d92b596d
Author: Jinsuk Kim <jinsukkim@chromium.org>
Date: Fri Jul 28 01:05:46 2017

Use UTF-8 as default encoding for JSON

Unlabelled document of content type application/json is UTF-8
by default as per RFC-7159#sectio-8.1. This CL takes it into
account when instantiating TextResourceDecoder.

BUG= 748440 

Change-Id: I4b18d9c1359eb9c8f0ff3e10fee89ad48d8c2e35
Reviewed-on: https://chromium-review.googlesource.com/587829
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490172}
[modify] https://crrev.com/0a6f6a195d44070c256172a2ca0c8353d92b596d/third_party/WebKit/Source/core/html/parser/TextResourceDecoder.cpp
[modify] https://crrev.com/0a6f6a195d44070c256172a2ca0c8353d92b596d/third_party/WebKit/Source/core/loader/TextResourceDecoderBuilder.cpp
[modify] https://crrev.com/0a6f6a195d44070c256172a2ca0c8353d92b596d/third_party/WebKit/Source/core/loader/TextResourceDecoderBuilderTest.cpp
[modify] https://crrev.com/0a6f6a195d44070c256172a2ca0c8353d92b596d/third_party/WebKit/Source/platform/loader/fetch/TextResourceDecoderOptions.h

FYI - the change will be visible on M62.