The behavior of "Sync another catalog" has dramatically changed recently (not sure when, but perhaps when the new sync architecture was introduced) and the new bavior has 4 major problems - I hope these are not "as designed" features.
My observations and testing of this was done on July 22 and July 23, 2024 using Window/10 with LrC 13.3.1 and LR 7.4.1
This information is related to syncing a new catalog. It does not apply to changing the sync catalog to one that is a copy, backup or an upgrade of the currently synced catalog. Rather it applies to changing the sync catalog to a catalog, or copies of a catalog, that did not get its start from the currently synced catalog and this includes the scenario of creating a new catalog and then importing to it from the currently synced catalog.
1) All images from LR/Cloud are added to the LrC catalog and are placed in the Xfer Folder in the LrC Folders Panel (this the folder designated in the "Lightroom Sync" tab of the "Preferences" dialog) whether or not the image was already in the LrC catalog
2) All images in LR/Cloud that were only Smart Previews are down loaded to LrC as Smart Previews. As such they are shown in LrC with a status of "Smart Preview where the image file is missing" as there was no actaul image to be placed in the Xfer folder. This is the case even if the Xfer folder had an original full size copy of the image known to LrC prior to the sync process.
In other words, the reset of the sync catlog process caused the full size physical image file on disk to be physically deleted. So, where the LrC catalog had a perfectly fine, non missing original file in the same folder where new LR/Cloud image will go which had previously been synced with LR/Cloud (placing a Smart Preview in LR/Cloud), L:rC now has only a Smart Preview with a missing image file listed in LrC and the phisical image file on disk was secretly deleted bythe process.
If you have a copy of that deleted physical file saved someplace (like in a backup) and re-place it in the Xfer folder, then LrC will see it and it will no longer be considered missing.
3) Where there had been edits done in both LR and LRC, the ones from LR supersede and negate those done in LrC. The history steps for the LrC edits remain in the History Panel but nothing below the “imported from LR Mobil” entry added by syncing the new catalog is applied to the image. In other words ONLY the edits done in LR/Cloud are on the image and none of the edits done in LrC are applied to the image. In the past, a VC was created for the images from LR.
4) Where LrC already had the original image in the catalog but not in the Xfer Folder, the sync process ignores the previous image and adds a duplicate image to the catalog and to disk. So, now you have the one originally known to LrC with LrC edits in one folder and a second version from LR/Cloud with the LR/Cloud edits in another folder. These two images are not connected in LrC in any way.
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PDF containing full description of testing and results can be found herehttps://drive.google.com/file/d/14UDkxnM6GNLUT7aBy-TWIYdwuCHI-E8K/view?usp=drive_link
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