I don’t see this behaviour here. The popup can appear on both screens, depending on where the client is.
But I do know there are problems with multiple screens when you connect a new screen (or reconnect it) while the client is running. Then the coordinate system gets messed up in some way, and all popups get displayed on the same screen.
Restarting the client helps in this case.
As screens rarely get reconnected in a production environment, it’s not a big issue for us.
(I assume you’re running Windows here, for Linux, it will depend a lot on the chosen window manager and its behaviour, so please tell what your setup is in that case).