I got this to work by downgrading dotnet-runtime and dotnet-host to version 2.0.7-1 from 2.1.0-3. I also downgraded mono from 5.12.0-260 to 5.12.0-226.
I would like to get to the bottom of this. Is it a Roon issue with compatibility of newer mono/dotnet or is it a buggery in the packaging at Arch Linux??