In 4.6 I had set all windows to open in the centre of the screen using the window manager tweaks placement tab in settings.

In 4.8, no matter what settings I use in this dialog, all windows (regardless of size) open in the top left corner of the screen (even when using the "Under the mouse pointer" setting).

When I installed 4.8, I deleted all my 4.6 settings and cache as there were a few other issues that this corrected, but the window placement issue persists.

I have tried adding a new user to my system so I am only modifying default 4.8 settings, and the window placement issue still persists.

Using a stock Arch install with no package modifications from the repositories.
The only DE installed is Xfce 4.8 along with the required Gnome libraries.

Cheers.