In case make group channel categorizing “Instrument Tracks” into “Add Group Channel to Selected Channels”,
Definitely the quality of sound which was through this “Group Channel” is bad relatively, compared to the quality of sound which was through just stereo-out.
Each quality of sound which created from “Cubase 11” and from “Cubase 12” (but same source) is defenitely have gap.
Due to this problem, I’m still using version of “Cubase 11” even if “Cubase 12” was released.
Please check it why this matter happened, and please update it fixing this problem as early as possible.
How would you describe “bad quality “ ?
To see if the audio quality differs you would need to render the the instrument alone and and when it goes through the group (at the same bit/SR of course) then align the result on two tracks and invert the phase on one. What you’ll hear will be the difference, but I doubt there will be any under normal circumstances.
In your case you probably have some plugins or channel settings that are different somewhere and you didn’t figured it out yet. The Group track simply cannot be of worse quality than direct output alone.