Sometimes, the notification for completion of copying files, etc., appears in the top-left corner of the desktop instead of in the bottom-right corner with the default horizontal bottom layout of the main panel. This appears to be a regression since either Plasma 5.10, or Frameworks 5.35, I'm not sure which. I never experienced it with 5.9.x/5.34. The problem does not occur every time, which makes it difficult to reproduce. Also, turning the notifications applet off and on, or moving it out of the system tray does not help. There was a similar case with Plasma 4: see bug #314831, but it seems that it was never fixed.
*** This bug has been marked as a duplicate of bug 381130 ***