P: Photos are marked for republish even though there is no change.

I noticed after updating my new v10 converted catalog that LrC was acting slow in the Library module, so I disengaged the usual culprit for this, the "Automatically write changes into XMP" setting. Then to my dismay, I noticed that it proceeded to mark the majority of the photos in the current grid view as needing a metadata update, even though the catalog metadata was fully saved to XMP prior to the upgrade, as far as I was aware.

Then I paged down, and the pattern repeated: Lightroom scanned all the now-visible photos and found that almost all of them also needed to be updated on disk.

And I did it again. And again.

Eventually I wrote a script to send "Page Down" events to the program periodically to ensure that Lightroom looked at every photo in the catalog, then let it run overnight with the library filter set to "Metadata Status = Up to date", so that it would give me the list of photos that need no metadata update. The next morning, I scrolled the grid back up to the top and let it go again, to catch any photos it missed on the first pass.

In the end, it marked over four-fifths of my catalog as out of date. This beggars belief, since I normally keep "Automatically write changes into XMP" engaged.

Then I did an experiment: I ran exiftool on one of the photos marked as still needing an update, saving the result to a text file, told Lightroom to save the metadata (⌘-S) and ran exiftool on the result, saving the output to a different text file, and diffed the two outputs, and found only timestamp and program version differences!

Observe:

5c5< File Modification Date/Time     : 2020:10:23 18:31:53-06:00---> File Modification Date/Time     : 2020:10:25 10:46:00-06:007c7< File Inode Change Date/Time     : 2020:10:23 18:31:53-06:00---> File Inode Change Date/Time     : 2020:10:25 10:46:00-06:0026c26< Instance ID                     : xmp.iid:9acec219-b6a4-4918-a592-9fc6a0ab3486---> Instance ID                     : xmp.iid:b5e76784-9638-44a1-a8b6-4e17799d003f28c28< Metadata Date                   : 2020:10:23 18:31:53-06:00---> Metadata Date                   : 2020:10:25 10:46-06:0040,41c40,41< History Instance ID             : xmp.iid:eac2ee0b-0b2d-4143-8da1-cafb424d66cc, xmp.iid:9acec219-b6a4-4918-a592-9fc6a0ab3486< History When                    : 2014:04:27 20:29:30-06:00, 2020:10:23 18:31:53-06:00---> History Instance ID             : xmp.iid:eac2ee0b-0b2d-4143-8da1-cafb424d66cc, xmp.iid:b5e76784-9638-44a1-a8b6-4e17799d003f> History When                    : 2014:04:27 20:29:30-06:00, 2020:10:25 10:46-06:00

For that particular file, the "Metadata Date" inside Lightroom is in July of 2019!

You may then wonder why the file modification time in the diff isn't in 2019, but instead two days ago. I dug into a recent pre-upgrade backup of my photos, and indeed, the prior file's mtime was in 2019. So, not only did LrC v10 decide it needed to do a bogus update to the file's metadata, it touched the file prior to actually being told it was okay to do so!

This upgrade has entirely invalidated all of my photo backups. Everything has to be backed up again, all because LrC is being stupid about touching files unnecessarily.

Surely the only defensible case for updating the application version number in the file's XMP metadata is that I've changed the photo, so now the program is properly reporting the last application to update the metadata?

I see other posts in the forum here on related topics, such as complaints that publish services are forcing a re-publish of unchanged photos. I'm posting this because I think I've diagnosed this to a deeper level than most users.