I just checked and it is working fine with archlinux updated to now. dotnet-runtime and mono are not dependencies of Roon.
There is in some cases a bug (not yet fixed upstream. there is a long discussion in Roon forums) due to which you must restart it after boot with systemctl restart roonserver to make it work.
P.S A clarification: roon is using mono in /opt/RoonServer/RoonMono/ provided by the package.
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