aiohttp requests fail when ClientSession is reused

I'm experiencing something weird creating a library that interacts with a third-party rest API. I'm fairly new to python and aiohttp but no amount of googling has given me any results.

I've noticed only the first request making with a client session works. Subsequent calls fail.

    session = ClientSession()
    things = await Thing.all(session, username="me@mail.com", password="123") # works
    things = await Thing.all(session, username="me@mail.com", password="123") # 500 error from server
    things = await Thing.all(session, username="me@mail.com", password="123") # 500 error from server

However using a new session every time works:

    things = await Thing.all(ClientSession(), username="me@mail.com", password="123")    
    things = await Thing.all(ClientSession(), username="me@mail.com", password="123")
    things = await Thing.all(ClientSession(), username="me@mail.com", password="123")

I'm not closing my sessions. There's nothing particularly weird about how I make calls from within my Thing class:

    resp = await session.post(
        url,
        data=f"Authorization=Bearer {token}" + data,
        headers={"accept": "application/json", "content-type": "application/x-www-form-urlencoded"}
    )
    resp.raise_for_status()
    return await resp.json()

It could very well be the API server's fault, it's not a great API to be honest, but does anyone have any clue of what could be leaking from one request to the next so sessions can't be reused?