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This issue is about how if a coroutine is wrapped in a Task with asyncio.ensure_future(), then portions of the exception chain can be lost.
Specifically, if you run the following code, then ValueError will be raised with exc.__context__ equal to the KeyError:
 import asyncio
 async def raise_error():
 raise ValueError
 async def main():
 try:
 raise KeyError
 except Exception as exc:
 future = raise_error()
 # Uncommenting the next line makes exc.__context__ None below.
 # future = asyncio.ensure_future(future)
 try:
 await future
 except Exception as exc:
 print(f'error: {exc!r}, context: {exc.__context__!r}')
 raise
 asyncio.get_event_loop().run_until_complete(main())
However, if you uncomment the `asyncio.ensure_future()` line, then the ValueError will be raised with no __context__.
I originally raised this issue a couple years ago here:
https://mail.python.org/pipermail/async-sig/2017-November/000403.html
There it was suggested that this was a special case of this issue:
https://bugs.python.org/issue29587
However, after writing code to fix that, this issue still exists.

My PR is ready for review: https://github.com/python/cpython/pull/19858 

Closing this as a duplicate of https://bugs.python.org/issue29587
It turns out that, as Nathaniel first suggested, this is really just another special case of that issue (the "yield from" case as opposed to the "yield" case). It's just that the particular examples provided in that issue were for "yield" and not "yield from", so it wasn't immediately evident.