I’ve tried other applications a well – same problem. And if I don’t use the @, all works fine: [emacs|aterm|xterm|gv|…] appears exactly where I want it to be (or where the placement policy puts it when I don’t specify a position.) But as soon as I use the @, the -geometry is ignored, and the window is placed according to the placement policy.
I belive that the @ in the geometry string is an fvwm-extension, and geometry options to programs are parsed and handled by them, and not processed by fvwm, to produce USPosition hints.