Using S1 v5.4.0.66465 with the ATOM SQ(1.17). Win10.
While triggering a vst instrument from the atom sq and the arpeggiator note fx is on and octaves are changed with the b-h keys on the sq while a chord is played, note(s) hang. The note hangs both aurally in s1 and visually on the atom sq.
I have not tried this with other midi controllers, so I'm not sure whether the lack of a note off is due to the sq firmware or s1.
I'm also not sure why a double or triple press on stop doesn't trigger an all notes off on all channels.(other programs have implemented this for quite some time 
)