Just a few remarks on Shared Albums, to explain the strange behaviour. What you have seen is the expected behaviour - not a bug, just a bit confusing, because the upload to the Shared Albums can be very slow. When we share photos to a new Shared album, it can take several hours until they will appear in the album.
You will be much better off with the Shared iCloud Photos Library, but you should be aware, that you will be currently only sharing the photos, not the albums, and the people we are sharing the library with need to have iOS 16, iPadOS 16, or macOS 13 Ventura or later installed. You will no longer be able to see the photos you share on devices with an older system version as the shared photos will be removed from your iCloud Photos Library to the Shared iCloud Photos Library. There is still a long way to go for the shared iCloud Photos Library, but a big step in the right direction.
Shared Albums are still the old Shared Photo Streams, introduced with OSX 10.8 Mountain Lion, to be used with Aperture or iPhoto, RIP. When Photos replaced iPhoto, the Shared Photo Streams have been renamed to Shared Albums, but they are still essentially the old streams and cannot be used like albums at all. They are a separate service from iCloud Photos, using a free part of iCloud, that does not count against our iCloud+ subscription. The shared albums are stored separately from our iCloud Photos Library and are shown in Photos but the photos do not appear in the Library albums "Days", "Months", Years", Media, Recents, etc. The search and the smart albums do not find these photos, the shared albums are holding own copies of the shared photos, specifically "optimized" for sharing - downsized to just 2048 pixel at the longest edge, and many metadata stripped. Shared albums are meant to be used to show our photos to others, without flooding the devices of the recipients with too many huge image files, and without giving away personal data, like the titles, keywords, or locations. The shared albums should be used like a chat, not as a web gallery - we drop a photo into the shared album, add a comment, the recipients comment on the photo, press the like button, add a photo of their on, we comment on their reply, etc. Essentially the Shared Albums are a predecessor of iMessage, fun to have, before I had an iPhone, but now they are a living fossile, just like My Photo Stream - yesterday's snow. Because the Shared Albums are discussion streams, they are sorted by the date we add a photo and we cannot sort them manually. Otherwise the discussion would lose the context. Last year Apple added the ability to sort shared albums by the date and stopped removing the capture dates from the shared photos, but this will not sort the versions the subscribers are seeing, the sorting is only locally on our device.
You cannot use the the Shared albums to transfer photos at the original quality, because photos will be downsized, and most of our work with tagging the photos will be lost. We cannot add adjustments to the shared photos in the shared album.
All this is much better in the Shared iCloud Photos Library.
