Dorico - Can't unmute in Play (Halion VST)

I am using Dorico 5.1 with Halion. I cannot unmute both channels – I can unmute one, but when I try to unmute the second, the first becomes muted again. Here is a screen recording (sorry it’s so jerky, but I’m using Windows’ snipping tool) : https://youtu.be/SQPGwkzzoxs

I have seen complaints about this behavior with other VSTs, but I still have not seen a solution.

Thanks.

@Ted_Trobaugh

Your video is marked private and inaccessible, at least that’s what I see on my end.