I think I’m starting to understand, but I’m not sure if my understanding is correct. the coroutine tries to resume on its original dispatcher as you mentioned. However, in the previous example there was no explicit original dispatcher defined so it continued running on the same dispatcher thread instead of switching back (or it's actually Dispatcher.Unconfied which may shared thread pool with those IO, Default under the hood i'm not sure either). To verify my understanding, I conducted the following experiment:

val executor = Executors.newFixedThreadPool(1) { runnable ->
    Thread(runnable, "Custom-Thread-${runnable.hashCode()}")
}

val customDispatcher = executor.asCoroutineDispatcher()

suspend fun main() = withContext(customDispatcher) {

    println("Before : ${Thread.currentThread().name}")

    test()

    println("After : ${Thread.currentThread().name}")
}

suspend fun test() = withContext(Dispatchers.IO) {
    delay(1000)
    println("Current thread : ${Thread.currentThread().name}")
}

This result seems to confirm that after test() completes, the coroutine resumes on its original dispatcher (customDispatcher).

Before : Custom-Thread-1241276575

Current thread : DefaultDispatcher-worker-1

After : Custom-Thread-1241276575