Tim, here’s what is probably going on:
NPP has two ‘view ports’. For my setup, it’s a top view and a bottom view.
If I open/create two files as you outlined, both files are in the top view port. If I move file1 to the bottom view and then move file2 to the bottom view, there’s nothing left in the top view, and it collapses and disappears. At this point the bottom view takes up the whole window and NPP looks just like it did when I started, but in actuality, both files are now in the bottom view and the top view is hidden. Now, if I move file1 to ‘other view’ what I’m really asking it to do is move file1 from the bottom view to the top view. What you THINK you’re doing is moving file1 to the bottom view, but it’s already in the bottom view. It just looks like file2 is getting moved but in reality it’s staying in the bottom view and the top view is uncollapsing and getting filled with file1.
If you move both files to top view (or maybe in your case the left view) the move behavior you’re expecting should be back.
TLDR: NPP has two views that stay put. If you move everything out of a view it will collapse, but it keeps the same place relative to the other view.