The claws-mail main window does not appear on the virtual screen that has been assigned to claws-mail by a style in fvwm. It appears always on the current virtual screen. On the contrary, sub-windows like Message Compose or adressbook do respect the fvwm styles. I remember that in some earlier version (maybe 2.7?) the behaviour was correct, and all windows of claws-mail always obeyed the fvwm styles file. I would like to start claws-mail on a certain virtual screen and to sit there without interfering with other screens on the virtual desktop.
I think you should look at what changed in fvwm, as, iirc, nothing changed in Claws.