Just upgraded to Cubase 8.5.15. I have projects with Halion Sonic, with multiple channels/instruments.
It used to be that I could happily mute/solo individual instruments and channels on and off. But now since the update, if I mute or solo an individual instrument, it also changes the overall Halion VST settings at VST instrument level. A little annoying as I can’t quickly mute/solo individual instruments any more. Well, I can, but when I un-mute/un-solo I also then have to go up to the main Halion channel and alter things there too.
Mute/Solo/Listen hasn’t been working right for some time. See this related thread: https://www.steinberg.net/forums/viewtopic.php?f=226&t=95392
Are you talking Instrument Tracks or MIDI Tracks sent to the instrument rack?
My advice is get onto support and make them investigate and hopefully acknowledge the problem and get it fixed.
Mute/solo is somewhat broken in Cubase. Sometimes you hit solo and random other tracks are soloed too. Same with mute. Some random connections are made. Then you click on solo on that other track, and it somehow resets it, and all is fine again. It’s been like that since version 7 afaik.