Why is ThreadStatic data being unexpectedly shared between threads?
Because that field isn't static. It only applies to static fields.
If this is 4.0, maybe look at ThreadLocal<T>
As @MarcGravell points out your field is not static, which is the cause of your problem.
However, if a consumer were to store the return from GetLoggingContext this value would be available to be shared between threads. I generally try to use any ThreadStatic variables as implementation details of a class, rather than exposing them outside of it.