New windows open in empty frame although window_open_frame is set to 'current'

Hi,

by default window_open_frame is set to 'current' and I doen't touch this in my musca_start.

What happens?
I open an application, then I split my screen in two frames.
With focus on the used screen I open a new application.
This new app goes to the empty frame but from what I understood this should only happen when window_open_frame is set to 'empy'

musca -c 'show settings'
set border_focus Blue
set border_unfocus Dim Gray
set border_dedicate_focus Red
set border_dedicate_unfocus Dark Red
set border_catchall_focus Green
set border_catchall_unfocus Dark Green
set border_width 1
set frame_min_wh 100
set frame_resize 20
set startup .musca_start
set dmenu dmenu -i -b -nf '#aaaaaa' -nb '#222222' -sb '#535d6c' -sf '#ffffff' -fn 'terminus-10'
set switch_window sed 's/^/raise /' | $MUSCA -i
set switch_group sed 's/^/use /' | $MUSCA -i
set run_musca_command $MUSCA -i
set run_shell_command sed 's/^/exec /' | $MUSCA -i
set notify echo `cat`
set stack_mouse_modifier Mod4
set focus_follow_mouse 0
set window_open_frame current
set window_open_focus 1
set command_buffer_size 4096
set notify_buffer_size 4096
set frame_display_hidden 1
set frame_split_focus current
set group_close_empty 0
set window_size_hints 1

 musca -c 'show frames'
0 * 0 22 840 1028 0
1 - 840 22 840 1028 0

Kind regards

Thomas