Which version of Photos are you using? Where is your Photos Library stored?
When I want to create a slideshow with the photos in a certain order, I arrange the photos in an album, then create the slideshow from the album and use a theme with only one photo per slide. Photos will shufffle the photos, if we drag photos into a slideshow project. I ctrl-click the album and use the command "Create Slideshow".
See: How to turn an Album into a Slideshow and preserve the Sorting,2019 Version
But if the slideshow is reverting back, after you open it again, the Photos library may be damaged and cannot be saved properly. Is the library in your Pictures folder or on an external volume?