dev-dotnet/newtonsoft-json-6.0.8-r1::dotnet (dep of dev-dotnet/1.0.27::dotnet, or I would have tried a more recent 7.0.1_p... or 8.0.1_p...) installs into /usr/lib64/mono/gac not /usr/lib/mono/gac, so it is not found when trying to emerge buildtools. Creating a symlink works, but shouldn't be necessary. I'm not quite sure whether this should block bug 506276, the tracker for profile 17.1.
Seems that package is no longer present (main gentoo or dotnet overlay) so I'm closing as obsolete.