I'm not really sure what the root cause of this is but I've had a similar problem that is resolved by using this bit of code instead of ensureDeferred

def as_deferred(f):
    return Deferred.fromFuture(asyncio.ensure_future(f))

I found this to be the best guidance about mixing twisted and asyncio
https://meejah.ca/blog/python3-twisted-and-asyncio