This is an issue that occured on a 2020 Apple M1 Macbook Pro, MacOS 11.5.2, with Lightroom CC version 4.4.
Last week I decided to switch from Lightroom Classic to Lightroom (4.4), because my photos were starting to take up too much local space. I followed every step of the migration instructions. The app told me everything had synced and at first everything looked exactly as I would expect. I even started a few fresh edits the day after, in Lightroom (4.4). Now, a week later, I open Lightroom CC and it shows only 26 photos, with no trace of any of the thousands of others. Clicking on the cloud logo in the top right, it tells me everything is synced, and I'm using 73,8 GB out of my 1TB of cloud storage. That's indeed about the amount of space that my full library used to take up, so I assume all my photos are present on Adobe's servers, but they're nowhere to be seen in Lightroom. Where did everything else go?
In the bottom left it shows albums, containing one album called "Migrated Lightroom Catalog-2-2-v10", supposedly only containing said 26 photos.
I should also mention that I found a Lightroom Library file of 76 GB in my bin (I did not put it there), and my Pictures folder now contains a Library file that's only 89,2 MB. But even when opening the file that somehow ended up in the bin, it only shows the same 26 photos, but no trace of anything else.
Is there any way I can force Lightroom to load the full library/all of the content present in my cloud storage? Is there something I should do with the library file that mysteriously showed up in my bin? Basically: where has everything gone, and how do I get it back?
When I log on there, it shows me the same photos under "all photos", but shows a list of 1180 photos under Sync Issues. It tells me to open Lightroom on my Macbook, which I did. But Lightroom on my MacBook doesn't mention these Sync Issues, and clicking on the cloud icon in the top right shows "Synced and Backed Up".
My recommendation is to open that Macbook and the Lightroom client on that Macbook that was originally used to sync, make sure it is connected to the internet and that sync isn't paused, and let it sit and sync for a while. I would give it several hours to see if any of the counts move.