Why ClientSession in async context manager freeze until timeout approximately once in 3-4 runs?

When I await any coroutine from response inside async context manager everything works normal but if I try to do it outside (or return response) it freeze?

Here is minimal representation of code

from aiohttp import ClientSession
import asyncio

async def fetch():
    url = ...
    headers = {"User-Agent": ...}
    parameters = [...]

    async with ClientSession() as session:
        session.headers.update(headers)
        response = await session.post(url, data=parameters, ssl=False)
        # await response.json() # <-- This line fix problem
    return response

async def main():
    r = await fetch()
    print(await r.json())

asyncio.run(main())

Problem fixed by returning response inside async context manager but I want to know why this problem appears.