Coroutine context lost easily between calls

[whirvis]

Describe the bug

When passing a Job as a CoroutineContext, the rest of the context is lost. I have a code fragment which produces unexpected output. When running it, I expect the following output:

CoroutineName(foo)
    CoroutineName(bar)
    CoroutineName(baz)
    CoroutineName(qux)
579

But instead, I get this:

CoroutineName(foo)
    DeferredCoroutine{Active}@2bea5ab4
    DeferredCoroutine{Active}@3d8314f0
    DeferredCoroutine{Active}@2df32bf7
579

After some digging, I realized this is because CoroutineContext.Element (implemented by Job) also implements CoroutineContext. In the implementation for CoroutineContext.Element, we find the following:

public override operator fun <E : Element> get(key: Key<E>): E? =
       @Suppress("UNCHECKED_CAST")
       if (this.key == key) this as E else null

Personally, I find this behavior very confusing and unexpected. Is there a reason why CoroutineContext.Element implements the CoroutineContext interface? (also: if the code fragment I gave is not optimal, please state so. I am new to Kotlin coroutines and I want to use them correctly.)