setting notifications to appear bottom right is broken on 4k monitors. the notifications appear as if they are appearing on a 1080p monitor which means they are about 2/3 from the left and top. this happens in both the standard and beta versions of the desktop app.
Do you have a multiple-screen setup? I am getting something similar with dual screens where if the Battle.net app is on my secondary monitor, that has a lower resolution, and toasts are directed to show on the primary monitor, they show on the primary monitor as if it has the lower resolution of the secondary monitor. It does this vise-versa as well.
You can open the notification settings of the Battle.net app and put your mouse over the Monitor selection dropdown for Toasts and it will show a red box around the area that it thinks is the full area of the monitor. And you can see there that it is taking the wrong resolution into account.