There's not really a second C# runtime. Java has several, some based on the Openjdk, but a few that are completely new like OpenJ9 and Graal.

The closest C# has is mono.

This sort of thing is bound to happen with that situation. Heck, it happens with C++ whenever a new C++ version comes out. Some C++11 features took years to make their way into all the compilers.

Graal is based on OpenJDK. OpenJ9 while using a separate JVM and JIT leverage the openjdk class path as well as the build environment and various other things.

I don’t think there’s a single alternate implementation that doesn’t leverage a good chunk of openjdk somehow