Photos “project" flips pictures after I flipped the individual photos.

I added photos to my library by taking a picture with my iPhone. The images transferred into the Photos app no problem but some were sideways or upside down. So I “flipped” them so they were upright.

Then I made a project out of them. BUT, the project ignores the flipping that I did.

I have tried flipping the pictures within the project. It shows the change on the thumbnails but not on the slide show. I have tried changing scaling photos to fit on the screen (on and off). And I have tried various themes within the photos app. None of these worked.

Can someone please help me so I can have a decent slide show for our 50th wedding anniversary? Thank you very much!