await vs asyncio.sleep: Does await keyword return control to the loop? Or maybe Pythons's asyncio StreamReader.read() is blocking?

Question

I encountered a strange situation. StreamReader in asyncio seems to be blocking.

received = asyncio.Queue()

async def read_loop():
    while True:
        received_data = await reader.read(4096)
        await received.put(received_data)

Then when I do

task = asyncio.create_task(read_loop())
# some time later....
task.cancel()

The task won't get cancelled. However, when I add one single line to this function:

async def read_loop():
    while True:
        asyncio.sleep(0)  # THIS LINE
        received_data = await reader.read(4096)
        await received.put(received_data)

Everything is working and the task gets cancelled.

It's like the await before reader.read doesn't return control to the event loop. I thought both await and asyncio.sleep return control to the event loop.

I want to understand why adding the sleeping function helped. I'd be grateful for help.