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This is not a new bug, but I finally decided to post this now, because I didn't see any other bug report regarding this.
If you play in borderless window mode and start the actual LoL game client the window position will be broken, if you focus any other window or your desktop while the small LoL splashscreen is still active. Then the main window will pop up and immediately move to the bottom right corner. I also experienced this behaviour on at least three different systems, so I doubt this is an issue of my home pc.
Finally you will end up with a window like this and you have to manually restore its position e.g. via taskmanager.