I can't drag and drop a photo that is in a shared album to a new position.
The photo just snaps back to its starting position.
iMac 21.5″, macOS 12.6
Question marked as
This is intentional. Shared Albums are just streams of photos for sharing, not really albums like the standard albums we can create on or devices. The photos will appear in the shared albums in the order we add them there, so the most recently added photos are easy to access at the end of the albums and the subscribers can find them on one place.
Shared Albums have not been intended to be used as a web gallery, they have been created as a chat with photos, long before we could share photos on an iPhone. When they first came out, the shared albums have been called "Shared photo stream". Apple renamed them to iCloud photo Sharing, and later "Shared Albums" when Photos for Mac came out. And now the users expect them to behave like albums, a bit confusing, as Shared albums are very different from albums. Typically we use shared albums to send a few new recent photos to someone. The person will reply by commenting on the photos, perhaps press the like button, reply by sending a new photo. Then we comment on the previous comments and the new photo, etc. This way the shared album will be a discussion about a sequence of photos, that would become disrupted, when we rearrange the photos. On macOS 13 Monterey or iOS 16 we can sort the photos in a shared album automatically by the date, but that will only sort them locally on our device, not on the devices of the subscribers. We can always revert to the order the photos have been added.
Shared Albums are not really a part of your Photos Library. They are using a different cloud service. While the standard albums are just referencing your photos in your library, the Shared Albums are creating and uploading separate copies of the your photos to iCloud. And these copies are downsized and optimized for sharing, some metadata are stripped for privacy reasons. This is mentioned on a footnote here: How to use Shared Albums in Photos on your iPhone, iPad and Mac – Apple Support (UK)
"Shared Albums upload a copy of your data. Downloaded content may not contain the same information as the original."