cubase 9.5.1 output disappear problem

does anyone have this problem ?

window 10 , cubase 9.5.1 , fireface ufx
there is no problem with audio interface.

if i make a group channel,
stereo output ( main output ) disappears with a 3~40 % probability .
so i go into audio connection - outputs , then stereo out button is missing

so i must restart cubase , then make a new stereo out(main out) bus and reset the track’s output one by one
so annoying …

help me plz …