Thanks for trying to help.
I am actually referring to Albums, though I called them folders. I contacted Support, and we worked out what was happening. I have about a dozen Albums, and within each Album I have multiple Albums, some nested to a third level of more Albums. The issue comes up when I select an album at a second or lower level which then shows all the images in that Album.
I then proceed to collapse that view at the top level, but am still displaying the images from the sub-album in the right-side window. If I then expand another top-level Album, it expands both the new and previous lists of Albums. Support suggested this was intended behavior, to show where you were, though agreed it was problematic given how I wanted it to work. The workaround is to select the top-level Album first, and then expand it to see it's sub-Albums. Not that big a deal, just an extra step that I think should be unnecessary.