I've taken some time to investigate this properly.
This problem will arise on an NVIDIA TwinView configuration when there is no GNOME panel at the top of the screen; either through being removed or moved elsewhere. notify-osd is programmed to follow the top panel only, quite explicitly and I believe this design was intentional. The fallback path attempts to place the OSD at the top-right of the desktop area - which spans all monitors in a TwinView configuration and hence appears on the rightmost monitor.
I am not sure what the correct solution is. I have made a private patch to fix this on my system but it is not suitable for wider release.
I've taken some time to investigate this properly.
This problem will arise on an NVIDIA TwinView configuration when there is no GNOME panel at the top of the screen; either through being removed or moved elsewhere. notify-osd is programmed to follow the top panel only, quite explicitly and I believe this design was intentional. The fallback path attempts to place the OSD at the top-right of the desktop area - which spans all monitors in a TwinView configuration and hence appears on the rightmost monitor.
I am not sure what the correct solution is. I have made a private patch to fix this on my system but it is not suitable for wider release.