Are you using the 32-bit version or the 64-bit version of Cubase?
Also what happens when you use your mouse a click a cymbal, which by default is assigned to channel 5. It should show up on channel 1. Does it show up left only on channel 1?
If you turn off downmix, You will lose the parts assigned to channels 2-4, and get kick and cymbals. As above, are the cymbals left channel only?
Also with downmix turned off, you won’t be able to hear ch 2-4, but you can still see them. Are they Left only? (i.e. is it really a downmix issue or something else)
Ron
Edit - I am using JBridge to bridge 32-bit to 64-bit, so I thought that might have something to do with it. So I removed the JBridging, but there was no change. Everything still works as expected.
On the surface this does not seem like a Cubase issue, but JS3 issue, because you are seeing the right channel missing in the internal JS mixer. So the question is why would it work in your other host? Try the things above and let me know.
Also - I tried to find your post on the rayzoon forum - could you point me to it? Usually Ralph is very responsive.