Same symptom here…but not yet found cure/solution.
Moving volume slider on track inspector doesn’t make any concurrent change to track volume or the level slider in HalionSonicSE. Moving the level slider in HalionSonicSE does correctly change volume though. Automation doesn’t work either.
If I substitute an older version of Halion/Sonic/SE (or any other VST instrument) into the same Cubase (10) arrangement, it miraculously works again.
I think that this is definitely points to something to with HalionSonicSE, specifically the version that is included in Cubase v10.x