不可取消的协程被取消

我正在尝试使用不可取消的协程，并编写了以下代码：

fun main(): Unit = runBlocking {
    // launch first coroutine
    coroutineScope {
        val job1 = launch {
            withContext(NonCancellable) {
                val delay = Random.nextLong(from = 500, until = 5000)
                println("Coroutine started. Waiting for ${delay}ms")
                delay(delay)
                println("Coroutine completed")
            }
        }

        delay(300) // let it print the first line
        println("Cancelling coroutine")
        job1.cancelAndJoin()
    }
}

输出：

Coroutine started. Waiting for 1313ms
Cancelling coroutine
Coroutine completed

到目前为止，一切都按预期进行。但是，如果我直接在函数中传递NonCancellable上下文（或更确切地说，Job）launch，则行为会发生变化，协程将被取消：

fun main(): Unit = runBlocking {
    // launch first coroutine
    coroutineScope {
        val job1 = launch(context = NonCancellable) {
            val delay = Random.nextLong(from = 500, until = 5000)
            println("Coroutine started. Waiting for ${delay}ms")
            delay(delay)
            println("Coroutine completed")
        }

        delay(300) // let it print the first line
        println("Cancelling coroutine")
        job1.cancelAndJoin()
    }
}

输出：

Coroutine started. Waiting for 4996ms
Cancelling coroutine

为什么第二个片段产生不同的输出？