shift-Fn keys shifted by two in tmux

[Marco]

Hi,

when I start mc in a terminal everything works ok. However, if I
start mc in tmux the shift-Fn keys are shifted by two. That means
that rename, which is shift-F6 usually, is shift-F8 under tmux.
And shift-F6 will edit the file in tmux.

Muscle memory doesn't pay attention to tmux running and I keep
launching the wrong operations. Why is that and how can it be
fixed?

Marco

GNU Midnight Commander 4.8.27
tmux 3.2a
TERM=rxvt-unicode-256color (terminal)
TERM=screen-256color (tmux)