Do you have an example of a site where this happens?

ifixit.com is one example.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/eb9e9ed60101406558df4ffcdfe20aee76730fb7

commit eb9e9ed60101406558df4ffcdfe20aee76730fb7
Author: Hui(Andy) Wu <wuandy@chromium.org>
Date: Thu Nov 16 19:59:00 2017

[Autofill] Move phone number rationalization to filling phase.

Current rationalization logic of phone numbers runs in fields detection
phase, where the fields' is_focusable state might be obselete. The
rationalization use this state to determine if the logic will run.

Some form have phone fields initally non-focusable, then present them
dynamically, the rationalization will skip those fields. Moving the logic
to filling phase should address this issue.

Bug:  784935 
Change-Id: I579a3dab6efad3c2f09a72562b8ae7ab4c376311
Reviewed-on: https://chromium-review.googlesource.com/768295
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Hui Wu <wuandy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517159}
[modify] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/BUILD.gn
[modify] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/autofill_manager.cc
[modify] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/autofill_manager_unittest.cc
[modify] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/form_structure.cc
[modify] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/form_structure.h
[modify] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/form_structure_unittest.cc
[add] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/rationalization_util.cc
[add] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/rationalization_util.h
[add] https://crrev.com/eb9e9ed60101406558df4ffcdfe20aee76730fb7/components/autofill/core/browser/rationalization_util_unittest.cc