Seems there are some SHARED ALBUMS issues in Photos for Mac.
I believe since recently updating to Tahoe 26.6.2 on my Macbook Air 13" M4 there is no longer "Activity" reported for my Shared Albums. I have close to 17k photos in iCloud, with lots of space available, although it might be helpful to know if there is an upper limit to number of photos (regardless of available storage). I have not checked yet to see if these symptoms appear on my other Apple devices.
That is: The Activity view under Shared Albums in Photos on Mac, displays "No Shared Photos or Videos. You can add photos of videos from other albums."
This occurs despite plenty of activity with new photos, comments, etc. on shared albums.
I have "rebuilt" my Photos using the Repair on startup of Photos. This did not fix the issue and now after several days there are still SHARED photos that are "stuck" in iCloud, showing no photo, just the little cloud icon. I believe ALL my Library photos are present locally and in iCloud, but the Shared Albums seem to be an issue. Similarly (and maybe a separate issue) I can no longer use Shared Albums as a screen saver, which I reported as a bug.
Any suggestions appreciated. TIA.