I believe Kakoune is hard-coded to set the window title to match the modeline. You can prevent Kakoune from setting the modeline at all with
set global ui_options ncurses_set_title=false, but you can’t change the content separately from the modeline.
You can also see the title inside tmux if you put
set-option -g pane-border-status top in your
.tmux.conf. If your terminal definition includes
hs capabilities (as putty and termite and kitty and alacritty do), tmux will copy the active pane’s title to the window title if you
set-option -g set-titles on.