Thumbnails disappear from project

Normally, dragging a thumbnail from one album to another copies the thumbnail to that album and leaves the other in place. If the album is in another project, the thumbnail is added to that project. I encountered a situation where thumbnails were deleted from the source project, although they remained in the source album.

In this case, a new album was created in a project and thumbnails found in a global keyword search were copied (by dragging) to the new album in that project. I did the search by selecting the name of the species from the list of Keywords in the Filter sidebar. Concurrently, those thumbnails disappeared from the project. I created a new project and moved the albums from the affected project to the new project and the thumbnails reappeared in the project.

Details: I have a large collection of bird images in my referenced file catalog. I decided to create new albums for each species in the Project labeled "Birds." I did this for a couple of dozen species. First, I searched for species by Keyword in ALL Images and copied them to the corresponding albums, but this was slow. So, I changed strategies. I have a smart album that has all images with birds as a keyword as well as another album in the Birds project labeled "Miscellaneous Birds." First I did keyword searches for each of the species in the Misc Birds folder, and then copied the found thumbnails to the new album for that species. I did the same with the Birds smart album. When I was done, the Project folder was empty, but the thumbnails in it contained all of the found thumbnails that I had copied from the found keyword search. I created a new project and dragged all of the albums in the old project to it and the thumbnails in the albums in the empty project reappeared.

This is not a huge problem if you know to expect it, but it does seem odd.