Have a look at this user tip: How to turn an Album into a Slideshow and preserve the sorting
Photos does not always respect the sort order in the album. Currently this should work:
- Select the sorted album in the sidebar, but do not select the photos inside the album, just the album.
- Ctrl-click or right-click the album in the sidebar, then use the "Create" command from the contextual menu, that will be shown after ctrl-clicking or right-clicking.
And you must not use the command "File > Create", it is working differently form the Create in the contextual menu. even dragging photos to the slideshow may mix up the order.
For example:
