https://chromium.googlesource.com/chromium/src/+log/63.0.3233.0..63.0.3234.0?pretty=fuller&n=10000 is the commit log, possible culprits include:

Improve download service scheduling logic crrev.com/26feda111405829458f98c85eee94b649583a669
Download Service: Handle transient download in target determiner.
 crrev.com/b8b90143fe7723118041beecd79d813923c509e7
Fix download resumption to handle cancel case crrev.com/222f6c8a6c9501abab1353501d662a5c403d06e6


I didn't see any releated offline pages changes in this release.

I don't think this is downloads related, as I'm seeing deletion logs in the offline event logger.  This is likely related to romax' cache directory change. Removing dtrainor.

Does this affect only pages saved before Chrome upgrade or the ones saved after the upgrade as well?

If only the former, than this is not a Pri 1 and there is a chance there is nothing to fix here.

no, this was ones saved after the upgrade if I recall correctly.

OK, scratch that, it seems that the patch I was thinking of has yet to land...

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9e20e7fb5b573f483739f27e51236241a62f57a3

commit 9e20e7fb5b573f483739f27e51236241a62f57a3
Author: Justin DeWitt <dewittj@chromium.org>
Date: Tue Oct 10 04:51:30 2017

[Offline Pages] Use proper archive directory for prefetching offline pages.

We hardcoded a directory that is no longer suitable for temporary pages.
The effect was that pages were deleted on startup.

Bug: 772908
Change-Id: Id9a9d1bc704cceffa7c3144735df60a449259b74
Reviewed-on: https://chromium-review.googlesource.com/707726
Reviewed-by: Jian Li <jianli@chromium.org>
Reviewed-by: Yafei Duan <romax@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507589}
[modify] https://crrev.com/9e20e7fb5b573f483739f27e51236241a62f57a3/chrome/browser/offline_pages/prefetch/prefetch_importer_impl.cc
[modify] https://crrev.com/9e20e7fb5b573f483739f27e51236241a62f57a3/components/offline_pages/core/offline_page_model.h
[modify] https://crrev.com/9e20e7fb5b573f483739f27e51236241a62f57a3/components/offline_pages/core/offline_page_model_impl.cc
[modify] https://crrev.com/9e20e7fb5b573f483739f27e51236241a62f57a3/components/offline_pages/core/offline_page_model_impl.h
[modify] https://crrev.com/9e20e7fb5b573f483739f27e51236241a62f57a3/components/offline_pages/core/stub_offline_page_model.cc
[modify] https://crrev.com/9e20e7fb5b573f483739f27e51236241a62f57a3/components/offline_pages/core/stub_offline_page_model.h