Hi & welcome aboard
How are these files organised in the album to begin with? Keeping photos in a certain order can be tricky if you rename photos from their default year/month/date/time format.
If the lecture photos are titled with numbers first 1. 2. 3. etc they should stay in order irrespective of which album they are sent to.
Are there many photos in this lecture? I was thinking to select them from the library and send them fresh to another album to test if they stay sorted or not.
Are you dragging them from the slideshow folder to a newly created folder?
I don't have any real up-to-date experience with Photos app as I stopped using it quite a long time ago.
Let us know if any of this helped you.
Cheers,
Marty