I have the same problem.
Version 61.0.3163.91 (Official Build) (64-bit)
Running on Korora 25 with kerne:  4.12.11-200.fc25.x86_64 

This is also reproducible in Mac. See attached screenshots.

Though the headers has the wrong content-type (text/html) instead of the correct (application/json), in v60 it was still correctly formatted.

Deleted: stable60.png 144 KB

	stable60.png 144 KB View Download

Deleted: canary63.png 158 KB

	canary63.png 158 KB View Download

Deleted: XHR_headers.png 124 KB

	XHR_headers.png 124 KB View Download

Able to reproduce on latest stable 61.0.3163.100, latest canary 63.0.3222.0 using windows 10, Ubuntu 14.04 and Mac 10.12.6 with the steps mentioned in original comment.

Manual Bisect Info:
================
Good Build:61.0.3143.0 
Bad Build:61.0.3144.0 

You are probably looking for a change made after 483126 (known good), but no later than 483127 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/b9546a7a38d574646e91e92c12d9247378b5bb11..147b1c8545c7b989df376d455642c47e01f40c22

Review URL: https://chromium-review.googlesource.com/552820

Suspecting same from changelog.

@eostroukhov: Please confirm the bug and help in re-assigning if it is not related to your change.

Thanks!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2a04495525d82b290a619c4abd62067e9fa79bad

commit 2a04495525d82b290a619c4abd62067e9fa79bad
Author: Nathan Bruer <allada@chromium.org>
Date: Thu Sep 28 23:42:01 2017

[Devtools] Regression: Fixed rendering of HTML and JSON in network

* Fixed bug where network preview would not render JSON if it was
fetched and and had an error code.

* Fixed bug where network preview would try to render as  HTML
agressively if requested via XHR.

R=pfeldman,eostroukhov
BUG= 765286 , 767393 , 763921 

Change-Id: I9a160e50241e406456623469738b0694800ae586
Reviewed-on: https://chromium-review.googlesource.com/687890
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Blaise Bruer <allada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505212}
[modify] https://crrev.com/2a04495525d82b290a619c4abd62067e9fa79bad/third_party/WebKit/LayoutTests/http/tests/devtools/network/network-choose-preview-view-expected.txt
[modify] https://crrev.com/2a04495525d82b290a619c4abd62067e9fa79bad/third_party/WebKit/LayoutTests/http/tests/devtools/network/network-choose-preview-view.html
[modify] https://crrev.com/2a04495525d82b290a619c4abd62067e9fa79bad/third_party/WebKit/Source/devtools/front_end/network/RequestPreviewView.js

This bug requires manual review: M62 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), bhthompson@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

allada@ - can you please provide more explanation for why this is required for M62 merge? Why can it not wait until M63? Since we're 2 weeks away from M62 stable, we're only taking critical and release blocking fixes. 

FYI re: M62 merge and a CM question
CR 767386 (dup of CR 763921) has user comments that (part of this) is already fixed in M62

This is primarily a devTools issue, developers can use Canary/Dev as a workaround.

The other developer WA is to use 'Content-Type: application/json'

CM question: Since the commit fixes 3 CRs, how are users/developers following the other CRs supposed to know to look here (767393) for the M62 Merge discussion?

#11 I am going to push to merge this issue. This is a fairly large regression for us and we would like to get it fixed asap. A large part of the web uses JSON in their XHRs and devtools will render most of them as HTML which is useless for JSON.

 Issue 770629  has been merged into this issue.

Ok Great thanks for more detail. If this has been well tested in Canary, approving merge to M62. (branch:3202)

 Issue 765081  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f674b40baf2d44d1b27f33b3ed39e60b2b9169da

commit f674b40baf2d44d1b27f33b3ed39e60b2b9169da
Author: Blaise <allada@chromium.org>
Date: Tue Oct 03 20:17:01 2017

[Devtools] Regression: Fixed rendering of HTML and JSON in network

* Fixed bug where network preview would not render JSON if it was
fetched and and had an error code.

* Fixed bug where network preview would try to render as  HTML
agressively if requested via XHR.

R=pfeldman,eostroukhov
BUG= 765286 , 767393 , 763921 

(cherry picked from commit 2a04495525d82b290a619c4abd62067e9fa79bad)

Change-Id: I9a160e50241e406456623469738b0694800ae586
Reviewed-on: https://chromium-review.googlesource.com/687890
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Blaise Bruer <allada@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#505212}
Reviewed-on: https://chromium-review.googlesource.com/698906
Reviewed-by: Blaise Bruer <allada@chromium.org>
Cr-Commit-Position: refs/branch-heads/3202@{#561}
Cr-Branched-From: fa6a5d87adff761bc16afc5498c3f5944c1daa68-refs/heads/master@{#499098}
[modify] https://crrev.com/f674b40baf2d44d1b27f33b3ed39e60b2b9169da/third_party/WebKit/Source/devtools/front_end/network/RequestPreviewView.js

 Issue 766229  has been merged into this issue.

Verified the fix on Mac 10.12.6, Win-10 and Ubuntu 14.04 using Chrome beta version #62.0.3202.45 as per the comment #0.
Attaching screen shot for reference.
Observed that JSON is formatted and collapsible as expected.
Hence, the fix is working as expected. 
Adding the verified labels.

Thanks...!!

Deleted: 767393.JPG 306 KB

	767393.JPG 306 KB View Download