Found "A" Fix
I had the same problem. I don't know why I even tried this but it seems to have worked.
After double-clicking the file I wanted to view and having it show up behind the window I clicked from, I noticed that the Photo Viewer was 'Maximized' to fill up my screen. I double-clicked the top window bar to 'Restore Down' which will show the window like it had been manually sized and placed where wanted on the screen. Doing that caused the window to become the top layer window as usual. Then, instead of using the red 'X' in the top right of the window to close it, I clicked on the top left of the window to bring up the window menu and clicked 'Close' from there. I double-clicked the exact same file and it suddenly opened...