When multiple tracks are selected, and a message such as
is sent, it only affects the 'active' track, and not (all) the selected track(s) (similar for /track/solo, /track/recarm, etc.). I think this is as a bug. Or is it perhaps by design (which would be weird: in REAPER multiple tracks can be muted, solo-ed, etc.; and tweaking volume / pan for multiple selected tracks does work as expected)?