|An image of the misplaced favourites bar|
This is not a solution for that but it works temporarily. Whenever the bar moves to the middle of the screen you can do the following to get it back.
Whenever the bar moves to the middle of the screen, press ALT + F2 and type: gnome-shell --replace
|Back in position|
Now the gnome-shell will restart and the favourites bar will be back in position.