Photos in shared albums not sticking to sorting

Hi - If I wanted them to be displayed as video or slideshow, I'd create that as a slideshow project in photos, export the video, then share that (max length 15 minutes).

Shared albums are very limited, and not much use for anything other than making pictures available for viewing by others. If you want anything more, including specific order of display, you need to be looking at other solutions. One option could be a shared iCloud drive folder, with the image files sequentially named.

Shared albums save photos in the sequence of addition to the album. After adding them in reverse order, how are you viewing the shared album? Photos assigns the sequence numbers in ordinal fashion (1, 2, 3....99), but web apps or non-macOS systems may evaluate the numbering differently, so you may see 1, 10, 11, 12-19, 2, 20, 21...

1. Instead of using a shared album, try exporting the album (original album with custom arrangement): File > Export > Export X Photos (or Unmodified Originals) > File Naming > File Name > Sequential > (assign a prefix) > Export. Save the photos in a folder. They'll be saved as Prefix1, Prefix2...Prefix94.
2. Since you have less than 100 photos, renumber only photos # 1-9 as 01, 02 etc.
3. Upload the exported folder to the web app, cloud service or computer where you earlier saw the wrong order.

See if this works?

Is you use the "Newest Date Added" sort order, the most recently added photos will appear at the end of the shared album.

But any sort order you specify will only be visible on your device, it does not change how the subscribers will see the shared album on their devices or in the web interface. When I share an album, I do not allow subscribers to add photos. If they could add a photo, it would change the sorting on the published web page, even a comment is changing the sort order. The photo with the most recent comment will appear at the end. I am also careful not to invite anyone at all, before the album is complete, or the subscribers will be harassed by a flood of notifications whenever I add an additional photo.