The form in question had one field with an autocomplete attribute. The extracted formless form was thus a 1-field form. When doing preview/fill the full form is compared to the extracted form and found to have a different number of fields, triggering an early exit from the fill/preview routine.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fcbff9947514c47d09446268b43d7f82bedd5378

commit fcbff9947514c47d09446268b43d7f82bedd5378
Author: Roger McFarlane <rogerm@chromium.org>
Date: Mon May 28 20:38:47 2018

[autofill] Add UMA counter for times unowned fields are not fillable.

The CL adds histograms and associated unittests to count the number of
times autofill is suppressed due to unowned (formless) fields being
restricted to pages with "checkout" related signals.

Bug:  841784 , 806987
Change-Id: Ied2f55234938793de3c5e7bce248e2cfbe80fb6c
Reviewed-on: https://chromium-review.googlesource.com/1073568
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562308}
[modify] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/chrome/browser/autofill/autofill_interactive_uitest.cc
[add] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/chrome/test/data/autofill/formless_all_autocomplete.html
[add] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/chrome/test/data/autofill/formless_no_autocomplete.html
[add] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/chrome/test/data/autofill/formless_some_autocomplete.html
[modify] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/components/autofill/content/renderer/form_autofill_util.cc
[modify] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/components/autofill/core/browser/autofill_manager.cc
[modify] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/components/autofill/core/browser/autofill_manager.h
[modify] https://crrev.com/fcbff9947514c47d09446268b43d7f82bedd5378/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7a263ce0a1053cd02e5e18513cf879939aecced6

commit 7a263ce0a1053cd02e5e18513cf879939aecced6
Author: Roger McFarlane <rogerm@chromium.org>
Date: Tue Jun 05 13:36:43 2018

[autofill] Add UMA counter for times unowned fields are not fillable.

The CL adds histograms and associated unittests to count the number of
times autofill is suppressed due to unowned (formless) fields being
restricted to pages with "checkout" related signals.

Bug:  841784 , 806987
Change-Id: Ied2f55234938793de3c5e7bce248e2cfbe80fb6c
Reviewed-on: https://chromium-review.googlesource.com/1073568
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#562308}(cherry picked from commit fcbff9947514c47d09446268b43d7f82bedd5378)
Reviewed-on: https://chromium-review.googlesource.com/1087007
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/branch-heads/3440@{#186}
Cr-Branched-From: 010ddcfda246975d194964ccf20038ebbdec6084-refs/heads/master@{#561733}
[modify] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/chrome/browser/autofill/autofill_interactive_uitest.cc
[add] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/chrome/test/data/autofill/formless_all_autocomplete.html
[add] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/chrome/test/data/autofill/formless_no_autocomplete.html
[add] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/chrome/test/data/autofill/formless_some_autocomplete.html
[modify] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/components/autofill/content/renderer/form_autofill_util.cc
[modify] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/components/autofill/core/browser/autofill_manager.cc
[modify] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/components/autofill/core/browser/autofill_manager.h
[modify] https://crrev.com/7a263ce0a1053cd02e5e18513cf879939aecced6/tools/metrics/histograms/histograms.xml