Are photos now deleted from all folders at once?

I'm on a MacBook Pro running High Sierra, using Photos v. 3.0.

It used to be that I created album folders by event and copied / moved the images from the Photos Library to those folders and the source photo remained in the Photos Library.

I did that this morning but moved /copied the images to the wrong folder so I deleted them form the event folder. I discovered that the photos were also deleted from the Photos Library. Is this new?

As an experiment, I created another event folder, moved/copied photos from the Photos Library to two different album folders and deleted one photo from folder #1. This deleted the photo from folder #2 AS WELL AS the Photos Library. That is scary to me, especially since I thought deleting photos from album folders would not affect the photos in the Library at all.

So now I have photos in the wrong album and can't remove them due to their being somehow "linked" to each other. Did I copy them incorrectly? Is there a solution?