Pictures ordered into a sequence in an album get reshuffled when transferred into a slideshow project

I have about 200 high definition photographs in a Photos 3.0 album on an iMac 27 (with High Sierra 10.13.6).

I edit them where necessary. It is easier for me (large thumb views) to arrange them into a preferred sequence now, in the album. However, when I select my prearranged photographs and transfer them into a new slideshow project - the photographs get reshuffled almost ideally :-(

I tried to transfer my prearranged photographs in small batches - all in vain.

I have been using Photos since the beginning and do not remember having any problems of this nature. The Help for Phots say: "To reorder photos, drag photos into the order you want them in the bottom of the window.". Does it mean that the only place photos can be reordered is "in the bottom of the screen" (in a Project) ?

Best regards. - Kris